{A Book revealed unto you}, meaning: the Quran.

{So let there be no constriction in your chest because of it}, meaning: doubt that it is from Allah.

Muhammad said: The root of al-haraj is constriction, and one who doubts a matter feels his chest constricted by it; therefore, doubt was termed haraj. {That you may warn thereby} against the Fire, {and as a reminder to the believers}, they are reminded of the Hereafter by it.