Overall meaning :
Addressing His Prophet, Muhammad, may Allah bless him and grant him peace, Allah Almighty informs him that He has revealed to him just as He revealed to Noah and the prophets after him, and as He revealed to Abraham, Ishmael, Isaac, Jacob, the prophets from among the descendants of Jacob sent to the twelves tribes of Israel (the Tribes), Jesus, Job, Jonah, Aaron and Solomon, peace be upon them all, and that He gave David, peace be upon him, a book called the Zabur.
The Almighty further informs him that He also revealed to messengers whose stories He has already related to him in the Qur’an before the revelation of this , as well as to other messengers whose stories He has not related to him in it, and that He spoke to Moses directly without any intermediary.
Allah has sent these messengers to His servants, He asserts, to give those who obey Him good news of absolute happiness in this life and in the life to come and to warn those who disbelieve in Him and disobey Him of utter misery in this world and in the hereafter, so that people will have no excuse before Allah after having sent them messengers to convey His message to them. Allah is Almighty, All-Wise.
He then reassuringly says to His Prophet, may Allah bless him and grant him peace: If people disbelieve you, then Allah bears witness that the Great Book—the Qur’an—He has sent down to you is the absolute truth, which He has revealed with His knowledge, and the angels also bear witness to the truth of what you have brought. Do not be saddened, therefore, by the unbelievers’ unbelief and faithlessness. It is sufficient enough that Allah testifies to the truth of what you have brought.