كَلَّآ إِنَّهَا تَذْكِرَةٌ فَمَن شَآءَ ذَكَرَهُۥ
Kallā innahā tadhkira Fa-man shāʾa dhakarah "No! It is a reminder. So whoever wills remembers it."
How kallā works was treated in the sections on al-Takāthur and al-ʿAlaq; I do not repeat it here. In summary: kallā is a "no" said most often not to a sentence but to a piece of conduct; it cuts off a claim that has been made not in words but in action.
The claim cut off here is this: this business runs according to who is listening. That was the hidden assumption of the logic of the gathering. Kallā rejects the assumption, and after it gives the ground of the rejection: this is a reminder.
Kallā is at the same time the sūra's first hinge. The incident ends, the subject changes. In the twenty-third verse a second kallā will come, and there too the subject will change. The skeleton of the sūra is divided into three parts by these two words.
The pattern. A verbal noun / noun on the tafʿila pattern: the thing that reminds, the means of reminding. The Qurʾān uses this word of itself in several places (al-Muzzammil 73/19; al-Muddaththir 74/54; al-Ḥāqqa 69/48).
What does the pronoun refer back to? Innahā — a feminine pronoun. The views: (a) to the verses / to the sūra; (b) to the Qurʾān (because al-sūra or al-āyāt is feminine); (c) directly to "this admonition". There is no difference in respect of meaning; all of them point to the text itself.
Tadhkira is not "instruction", not "informing", but reminding. And reminding is the waking of something already present in the one addressed. To one who does not know, knowledge is given; to one who knows, a reminder.
This is one of the ways the Qurʾān defines itself, and it has consequences: if the text is a reminder, then there is something inside the one addressed that answers to it. At al-Shams 91/8 it was said, "and He inspired it with its fujūr and its taqwā"; the inspiration there and the reminding here look to the same place.
And its function in the context: reminding depends not on the quality of the one who reminds but on the readiness of the one who remembers. The weight of a gathering reminds no one of anything. For this reason who is listening does not change how the text works.
A grammatical nicety. In the preceding verse the pronoun was feminine (innahā), here it is masculine (dhakarahu). The reason: the pronoun now refers back not to tadhkira but to dhikr / the Qurʾān (masculine); or else the word tadhkira is taken in the sense of "admonition, counsel" (al-waʿẓ, masculine). A small detail, but it shows the scrupulousness of Arabic about pronouns.
مَن شَآءَ — whoever wills. One of the sūra's most loaded expressions, and it says something that needs saying once more: the door is open; whoever passes through, passes through.
The sharpness of this in the context needs to be seen. The Prophet was occupied with forcing a door — the door of the leading men. The verse says: forcing the door is not your affair. Whoever wills, enters.
And the exact counterpart of this is found at al-Takwīr 81/28-29: "For whoever among you wills to go straight" and immediately after it: "And you do not will unless God, the Lord of all the worlds, wills." The two sūras take the same two steps: first man's willing, then the place on which the willing rests. ʿAbasa stops at the first step; al-Takwīr adds the second. The two should be read together — this link will be treated separately in the section on al-Takwīr.
A caution. If this verse is read as "whoever wants to, believes; whoever does not want to, does not; and that is all there is to it", the meaning of delivering the message is emptied out. The sūra itself does not permit this: for forty-two verses it keeps up the reminding, repeats the call, sets out the evidence. So the sentence "whoever wills remembers" does not lift the call; it lifts the outcome of the call off the back of the one who makes it. A continuation of what the seventh verse said.