Al-Kawthar is the abundance of all goodness, foremost among which is the river in Paradise.
This speech is directed to our Messenger (saws), Allah (swt) mentions His great bestowment on His honorable Messenger (saws).
This Surah is one of Makki Surahs in which Allah (swt) wills to return the earthly measures to the Divine ones and the humans standards to the Divine standards, as the humans’ standards are plenty and conflicting while the Divine standards are one, the method is one, there is not any conflict between them.
The revelation reason of this Surah becomes clear to us when we understand the nature of the Arabic life and the Pre-Islamic practices. People at that time were very proud of boys as they are the ones who will keep their descendants and remembrance after their death. Thus, it was common to say: Who has no child, had no one to remember him. Consequently, they cared about lineage and saved it.
However, when the Prophet’s (saws) male children died, the disbelievers were happy because his (saws) lineage was cut, thinking that the message would then end as a result. This is their concept, whereas of Allah (swt) standards, the lineage is not connected with humans’ lineage. For the lineage of the Prophet (saws) is his nation’s children and his followers who obey his orders even after his death till the Last Day. That is why one of the orientalists expresses the truth by his saying: “I am very surprised that a man like Muhammad is still governing millions of people even though he is dead in his grave.” This is his point of view, while for us, we know this truth since Allah (swt) says:
﴾وَاعْلَمُوا أَنَّ فِيكُمْ رَسُولَ اللَّهِ ﴿
(And know that among you is the Messenger of Allah.) (Al-Hujurat: 7)
Thus, Allah (swt) willed to please His Messenger (saws) and soothe the effect of the saying of his society, so He (swt) said to him: (Indeed, We have granted you, [O Muhammad], al-Kawthar). These words are of Allah’s (swt), the granter, the Messenger (saws) is the receiver of this speech and he is the granted, the grant is Al-Kawthar. So what is Al-Kawthar?
الْكَوْثَرَ (Al-Kawthar): The Arabic word means the abundant good.
Thus, it concludes the plentiful good in all the aspects; money, prestige, health and children. So Allah (swt) has given the Messenger (saws) an abundance of every good that could ever be given.
Anas Ibn Malek (may Allah be pleased with him) narrated that:
“One day the Messenger of Allah (ﷺ) was sitting amongst us that he dozed off. He then raised his head smilingly. We said: What makes you smile. Messenger of Allah? He said: A Sura has just been revealed to me, and then recited: In the name of Allah, the Compassionate, the Merciful. Verily We have given thee Kauthar (fount of abundance). Therefore turn to thy Lord for prayer and offer sacrifice, and surely thy enemy is cut off (from the good). Then he (the Holy Prophet) said: Do you know what Kauthar is? We said: Allah and His Messenger know best. The Prophet (ﷺ) said: It (Kauthar) is a canal which my Lord, the Exalted and Glorious has promised me, and there is an abundance of good in it. It is a cistern and my people would come to it on the Day of Resurrection, and tumblers there would be equal to the number of stars. A servant would be turned away from (among the people gathered there). Upon this I would say: My Lord, he is one of my people, and He (the Lord) would say: You do not know that he innovated new things (in Islam) after you. Ibn Hujr made this addition in the hadith:” He (the Holy Prophet) was sitting amongst us in the mosque, and He (Allah) said: (You don’t know) what he innovated after you”
However, the interpreters differ in regard of Al-Kawthar meaning. Ibn Abbas (may Allah be pleased with him) said: Al-khawthar is something very great and plentiful and this is one of it. But why did they differ in interpreting this word?
Because if they were to interpret it as Prophethood, but the Messenger has known that he is a messenger, on the other hand, if they were to interpret it as the Qur’an, but the Messenger was the receiver of the Qur’an and the first one to be affected by it and if they said that it means the raising of the Messenger’s remembrance in the two testimonies , but it is well known to the Messenger of Allah (saws).
However, the Messenger (saws) interpreted Al-Kawthar as a bestowment in the hereafter, because it is a hidden matter, so the Messenger (saws) did not interpret it by a seen matter, but this does not mean to ignore its meaning of abundant goodness. For we do really know that when Allah (swt) says (We) then, after it, certainly, there is a great matter of great grant, because this pronoun indicates glorification, as when Allah (swt) says:
﴾إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَإِنَّا لَهُ لَحَافِظُونَ ﴿
(Indeed, it is We who sent down the Qur’an and indeed, We will be its guardian.) (Al-Hijr: 9)
Because the Holy Qur’an is a great matter.
Thus, when Allah (swt) says: (We have granted you). Definitely, this grant is great, in parallel with the greatness of the Granter (swt) and His capabilities in His dominion. So, you can imagine its greatness, in addition to conclude the river in the Paradise.
 Sahih Muslim, Book of Salah, hadith No. 400.