In the prayer, after Surah al-Fatihah, one of the surahs of the Qur’an must be recited. The person praying might choose any surah; of course, with the exception of the four surahs in which prostration is obligatory.
Among the surahs recited in prayer, Surah al-Ikhlas (or at-Tawhid) is preferable and in the traditions it is thus enjoined: “You have to read this surah in at least one of the rak‘ahs of prayer everyday so as to be among those who are praying.”
In terms of importance, this surah is equal to one third of the Qur’an, nay one third of the Tawrah, Injil and Zabur. If this surah is also recited after the prayer as a prayer supplement, God shall give the person the goodness of this world and the hereafter.
This surah is short, yet its content is very sublime. As Imam as-Sajjad (‘a) said, “Since He knew that meticulous and insightful individuals shall emerge in the future, God revealed this surah as well as the initial verses of Surah al-Hadid.
Recital of this surah is enjoined not only in prayer. In fact, frequent recitation of it removes the evil of tyrants and ensures one’s house from mishaps and perils.
Sa‘d ibn Mu‘adh, one of the companions and commanders of the army of the Messenger of Allah (S), was buried in the cemetery of Baqi‘ in Medina. The Prophet (S) participated in the burial rites for him barefooted and saying: “Ninety thousand angels have come to participate in the burial of Sa‘d.” The Prophet (S) asked Jibra’il (Archangel Gabriel) (‘a): “What is the reason behind your coming down as well as that of all these angels for the burial of Sa‘d ibn Mu‘adh?” Jibra’il (‘a) replied: “In whatever state—standing, sitting, riding, or walking—he used to recite the surah, “Say, ‘He is Allah, the One’.”
Concerning the circumstances surrounding the revelation of this surah, we read that the Jews, Christians and the idol-worshippers asked the Prophet (S) about God: “Describe your God to us.” In reply to them, the Prophet (S) recited this surah. This surah serves as the “identity card” of God.

“Qul huwa’llahu ahad”
Tawhid {monotheism} is the most fundamental principle of all heavenly religions and the prophets (‘a) have come to eliminate the effects of polytheism, disbelief and idol-worship and to invite mankind toward God.
Tawhid is the spirit and kernel of the teachings of the prophets (‘a). Not only beliefs but also laws and morality revolve around the axis of tawhid.
Tawhid is the frontier between faith {iman} and disbelief {kufr}, and entering the fortress of faith without acknowledgment of tawhid is not possible: “Say, ‘La ilaha illa’llah {There is no god but Allah}’ to attain success” and “La ilaha illa’llah is My fortress; so, whoever enters My fortress is safe from My wrath.”
This surah consists of the purest monotheist beliefs; hence, this surah is called Surah al-Ikhlas {Chapter: Purity}. This surah refutes the Christian belief in the Trinity, the polytheism of the Jews as well as the belief of the Arabs during the pre-Islamic period of ignorance who regarded the angels as the daughters of God.
Tawhid means the purification of mind and action from any form of polytheism and associating partners with God; neither theoretical polytheism nor practical ostentation. Instead, both the motive and the action itself must be only for the sake of God.
That is, He is One and has no substitute; He has neither similitude nor equal; He has neither limbs nor parts.
That is, He is the Worshipped Being {ma‘bud} who is unique in everything. Thus, His Essence is beyond the comprehension and understanding of man.
The reason behind His Oneness is that if there were also another “God”, this “God” would also have been supposed to send prophets to mankind to recognize and obey “Him”!
The reason behind His Oneness is that at the time of danger, all human beings focus only on the One Being and their hearts testify to the fact that only One Being gives hopes to man amid all difficulties.
The reason behind His Oneness is the harmony between the heaven and the earth, the universe and man, and the precise and orderly relations among all creatures. If you would ask three painters to draw a single rooster, for example, asking one of them to draw its head, the second one to draw its body, and the third one to draw its tail or feet, there would be no harmony among the three drawings when you place them together; one might be big while the other one might be small; one may have a dark color while the other one may have a light color.
Yes, the harmony existing between the sun, moon, earth, water, wind, soil, mountain, desert, sea, and others, on one hand, and the needs of man, on the other, signifies that there is only One Creator.
The human being inhales oxygen and exhales carbon dioxide. On the contrary, a plant absorbs carbon dioxide and releases oxygen. In this way, the needs of man and other livings creatures are met, and this harmony is the secret behind the life of men and plants.
The needs of an infant are met through the love and affection of its parents, and the weariness of the day is removed through sleep at night. The water of the eye (tears) has been created salty and the water of the mouth (saliva) fresh. In this way, the salty water cleanses the eyes while the fresh water prepares the food for the processes of chewing and digestion.
Instead of dying, the infant is taught to suck and even before it is born, its milk has been made available in the breasts of the mother. The food of some birds is located along the teeth of whales in the sea. And everyday, animals are satisfactorily provided with food.
During the Battle of Jamal, a Bedouin Arab asked Hadrat ‘Ali (‘a) about the meaning of tawhid. The soldiers complained to him, saying that it was not the time for such questions. Nevertheless, the Imam (‘a) explained to him the meaning and interpretation of tawhid, and then said: “It is for this very meaning (of tawhid) that we are now fighting.”
Yes, the struggle of the followers of truth throughout history has been for the sake of hoisting the banner of tawhid.

“Allahu’s-samad” {Allah is the All-embracing}
“Samad” means impenetrable, indestructible and unchangeable.
He is samad. So, He is not material or corporeal because all corporeal things experience change and destruction with the passage of time. Thus, He has no body that can be seen by the eyes or such an attractive force that, though unseen, has corporeality.
He is samad, the Impenetrable Force whose will pervades and penetrates all things.
He is samad, the Indestructible, the Honorable from whom all honors come and whatever honor and power anyone has emanates from Him. He is not in need of anybody or anything but everything is in need of Him.
He is samad, the Absolute Perfect Being, nay the Absolute All-perfect Being who possesses all excellences at the apex of their perfection. In order to attain perfection, all beings are in need of His grace, but He is and will not be in need of any being. His Command is above all other commands and His Will is above all other wills. He is in need of neither sleep nor assistance and partners in doing His work.
He is samad, and in a nutshell, He is the Needless God who is always needed by all.

“Lam yalid wa lam yulad” {He neither begat nor was begotten}
He is the Creator of all beings and is not the one who gave birth to them. His work is not reproduction such as “reproducing Himself”. It is instead bringing something into being from non-being or nothing.
In the case of the mother who gives birth to a child, the infant belongs to her genus and is like her, i.e. a human. But with respect to God, it is impossible for an equal and similar being to be begotten by Him or for Him to have been begotten: “Nothing is like Him.”
This sentence is against the belief of the Christians who believe in ‘Isa (‘a) as the son of God, and treat him like God. In the same vein, this verse negates the belief of the polytheists who imagined that the angels were the daughters of God by saying: “God has never begotten a child, whether a girl or a boy!”
Nothing is to be begotten unless that the one who begot is preeminent and superior to the begotten one, not to mention the fact that the one who begot existed prior to the begotten one!
His existence is not like the sprouting of the fruit from the flower; the tree from the seed; the water from the cloud; the fire from the wood. It is not like the emergence of words from the mouth; or the script from the pen. It is not like the emission of scent from the flower, or taste from food. It is not like the flashing of an idea from the mind, or intuition from the heart.
It is not like the appearance of heat from fire or cold from snow. He exists but is similar to nothing and no one. He is neither in anything nor is anything in Him. His relationship with things is not like that of a parent to his child. It is rather the relationship of the Creator {khaliq} with the creature {makhluq}.

“Wa lam yakun lahu kufuwan ahad” {Nor has He any equal}
He has no equal in being, perfection and action. He is One and no one is similar to Him. He is the One and Only and He has neither spouse nor offspring! He has no equal to be His assistant and partner.
How could man dare to regard His creature as His partner and commit this grievous inequity against Him?
“Polytheism is indeed a great injustice.”
O praying one! Do not consider others as His partners and associates in the favors that He bestows upon you, nor attribute to other than Him the things you do. Why do you think of attracting the attention of those who are like you - weak and needy? Try to win the favor of God instead who has no equal, weakness and need.
At the end of the surah, we shall take a cursory glance at its sublime content:

قُلْ هُوَ اللَّهُ أَحَد

That is, He is One in Essence as well as in Attributes. So, He is also One in worthiness to be the Worshipped Being {ma‘bud}.

اللَّهُ الصَّمَدُ

That is, He is free from want and all things are in need of Him, and He is Alone in being free from want.

لَمْ يَلِدْ

That is, He does not reproduce an equal for Him to have a peer and a match.

وَ لَمْ يُولَدْ

That is, He is the First and the Last. He is not created to have emerged from something.

وَ لَمْ يَكُن لَّهُ كفُواً أَحَدُ

That is, He has neither equal nor spouse; neither peer nor partner.
This surah negates all layers of polytheism, superstitions, delusions, and deviant beliefs from the Sacred Essence of God, while presenting to us pure and pristine tawhid.
According to the traditions, the verses of this surah are the interpretation of each other. They describe God in a step-by-step process:

First step: Say, “He is my God. He is beyond the comprehension of the human mind and intellect, and He is hidden and absent from any vision.”
At this stage, all the attention is focused on the Essence of God and not on His Attributes. The Essence alone is enough for His being the Beloved {mahbub} and the Worshipped Being {ma‘bud}. Hadrat ‘Ali (‘a) says:

وَ كَمالُ الاِخْلاصِ نَفْىُ الصِّفاتِ عَنْه

That is, perfect sincerity is that you worship Him without taking into account His Attributes. That is, you worship God for the sake of Himself and not on account of the favors that have been given to you.

Second step: “He is Allah, the Worshipped Being who possesses all perfection.”
In this stage, the Essence and Attributes are considered together. “Allah” is the Essence that possesses all the good Attributes, and thus He is worthy of worship and devotion. As the Qur’an states,

وَ للَّهِ الأَسمَاءُ الحْسنى فَادْعُوهُ بهَا

“To Allah belong the Best Names, so supplicate Him by them.”
Knowing God through the Attributes is the second step in which “Allah” is the comprehensive word for all those Attributes.
Considering God by way of the Attributes is a means that is manifested in the supplications, especially in the Du‘a’ Jawshar Kabir where we address God through a thousand Attributes.

Third step: “He is One and Unique in Oneness.”
In this stage, tawhid is discussed - tawhid in Essence and Attributes. His Essence is One and His Attributes are Unique and Unparalleled. The existence of His Essence and His Attributes are one and the same; not that His Attributes are a separate existence added to His Essence.
He is One who has no second or third. The numerical “one” {wahid} is different from oneness as a qualitative concept {ahad}. The Qur’an says: “God is ahad and not wahid.”
He is “One” (ontologically) and not “one” (numerically) which may have a second or a third of its kind.

Fourth step: “God is free from want.”
In this stage, the freedom from want which is the pivot of the Divine Essence and Attributes is an important description of God. In addition, it is not just in the form of a predicate, stating: “Allahu samad”. It is rather in the form of a fixed description of “Allah” and because of this, the word “Allah” is hereby repeated: “Allahu samad”.
He is One but not in need. Other than Him are many, but all the needs and attention of all those who are needy are geared toward that Independent One!

Fifth step:

لَمْ يَلِدْ وَلَمْ يُولَدْ  وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ

This stage, according to the traditions, is the interpretation of “الصَّمَد”. He is not in need of begetting a child. Neither is He in need of a father and a mother to beget Him, nor of a spouse, associate and partner to assist Him in His work.
If He were ever born, then He would not be the First, and if He ever gave birth, then He would not be eternal because He would be prone to diminish and dwindle. If He ever had an equal and a peer, then He would not be unrivalled. He is free and exempt from all these things.

سُبْحَانَ اللَّهِ عَمَّا يُشرِكُونَ

“Clear is Allah of any partners that they may ascribe {to Him}!”