Overall meaning :
Allah Almighty opens this great surah 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! Then He explains that these lofty and sublime verses are indeed verses of the perfect Qur’an that is full of wisdom.
The Almighty then says: How can the unbelievers from among the Quraysh and the Arabs find it strange that He has revealed to a man from amongst them—Muhammad, may Allah bless him and grant him peace—to warn all mankind of Allah’s punishment for unbelief and disobedience and to give the believers the good news of a rich reward for their good deeds? Even though We have sent revelations to a man from their own midst with the message to warn them and give them glad tidings, the unbelievers simply said about this messenger that he is clearly a mere sorcerer.