Overall meaning :
About what are the Quraysh idolaters asking one another?! They are asking one another about the momentous news (that is, the Qur’an and the facts of monotheism and resurrection, among other things, that it contains) over which they disagree, with some believing in it while others denying or doubting it.
No indeed! They will know with certainty the truth about what they deny as well as the consequences of their unbelief. Again, no indeed! They will certainly know the truth of the Qur’an and the resurrection which they deny.