Overall meaning :
The idolaters say, ‘When will this threat of punishment be fulfilled, if what you say is true?’ Say to them, Muhammad, ‘It may well be that some of the punishment that you demand to be hastened is close at hand.’. Surely your Lord, Muhammad, is gracious and merciful to people for not hastening their punishment and is so bountiful to them despite their wrongdoing, yet most of them do not give thanks to Him for the blessings He has bestowed upon them.
Certainly your Lord, Muhammad, knows all that their hearts conceal, as well as all that they say and do in public, and he will repay them accordingly. There is nothing that is hidden in the heavens and the earth but Allah knows it, and it is all recorded in the Preserved Tablet