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!
The Almighty then explains that this book—the Qur’an—which He has revealed to Muhammad, may Allah bless him and grant him peace, is a great book indeed, whose verses are perfected such that they are neither defective nor do they contain any falsehood whatsoever, then are expounded in detail through true accounts and commands and prohibitions; it is undoubtedly from Allah, the All-Wise, the All-Aware who disposes of all affairs and knows their consequences and outcomes perfectly well, so that you worship none but Allah and not associate any partners with Him.
Then He commands His Messenger, may Allah bless him and grant him peace, to say to the people: I have been sent to you from Allah to warn you of His punishment and to give you good tidings of His reward. Ask your Lord for forgiveness and turn to Him in penitence. He will let you enjoy a good life until your appointed time of death, and will graciously and fully reward those have merit by good words and deeds. But if you pay no heed and turn away from what I am calling you, I fear for you the punishment of a mighty day—the Day of Judgement. To Allah will you all be returned after death, so beware of His punishment; and He has power over all things: He is perfectly capable of resurrecting you, gathering you together for judgement and repaying you according to your deeds. These idolaters conceal their unbelief in an attempt to keep it secret, thinking that Allah is not fully aware of what they hide in their minds. Surely, even if they cover themselves with their garments, nothing is concealed from Him and He knows what they conceal and what they reveal, He has full knowledge of their secrets, intentions and innermost thoughts.