Tafsir of Qaf 50:45

{We are most knowing of what they say} A threat to them and a consolation to the Messenger of Allah (peace and blessings of Allah be upon him).

{...by a tyrant (jabbār)} Similar to the Almighty’s saying: {by a controller (musayṭir)}. It means: you are not one who compels them to believe; you are only a caller and a messenger.

It is also said: It is intended to mean showing forbearance toward them and abandoning harshness against them.

It is also permissible that it is derived from "compelling someone to do a matter" (jabarahū ‘alayhi), meaning: You are not a guardian over them to compel them to believe. The preposition ‘alā (upon) is in the position of its usage in your saying: "He is ‘alayhim (over them)," when one is their guardian and the master of their affairs.

{...who fears My threat} Similar to the Almighty’s saying: {You are only a warner to those who fear it} (An-Nazi'at: 45), because the warning only benefits such a person, not the one who persists in disbelief.

From the Messenger of Allah (peace and blessings of Allah be upon him): "Whoever recites Surah Qaf, Allah will lighten for him the agonies and intoxications of death."