Overall meaning :
This great surah opens with the 'disconnected letters’ to establish the miraculous nature of the Qur’an, which conclusively prove the Arabs’ total inability to challenge the Qur’an by producing something like it, even though it is composed of the very Arabic letters they speak!
Allah Almighty then addresses His Prophet, Muhammad, may Allah bless him and grant him peace, saying that this Qur’an is a divine book that He has revealed to him. He instructs him not to be distressed by conveying it for fear people might not believe it, and not to doubt that it has been sent down by Allah, Exalted and glorified be He, in order for him to warn the unbelievers and admonish the believers.
Then He commands people to follow the Qur’an, which has been revealed by Him, and to follow no masters other than Him or besides Him, stating that little do people reflect, call themselves to account and turn to the truth.