سُبْحَانَ ٱللَّٰهِ
Holy Qur'an
Al-Qur'an
Kids Qur'an
I.e., it has come down to man, through the Prophet, without any alteration, omission or addition.
Lit., "with truth" or "in truth".
The Qur-an was sent down by Allah in Truth: it was not forged by any mortal. It has descended in Truth: it was not and has not been falsified or corrupted in the process of being communicated to mankind.
The part of the Prophet was that of a Messenger: he was not responsible if the ungodly rejected it. He fulfilled his mission in promulgating and explaining it and leaving it as a legacy to the world.