Overall meaning :
Allah Almighty opens this noble 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 declares that these are the verses of the Qur’an, which is clear in every respect, in terms of its provisions, expressions and the different types of meanings its words carry; it is also clearly the truth and is the Clear Book that makes plain what is unclear and ambiguous. The Almighty further declares that He has sent down this Qur’an in the Arabic tongue—the language of the Arabs—so that they may understand it and obediently follow His guidance laid down in it. Then He addresses His Prophet, Muhammad, may Allah bless him and grant him peace, saying: By revealing this Qur’an to you, We relate to you the best of stories, and you were indeed before this Qur’an revealed to you among those who did not know anything about such accounts and totally unaware of them.