وَما هُوَ بِالهَزلِ
And it is not a jest.
EXEGESIS
Again, depending on which interpretation we preferred for the previous verse, it would be referring to either the Quran or resurrection. In either case, the message would be that neither of those things is a trifling matter that should be ignored or taken lightly.
Hazl literally means to be skinny and weak, and in usage denotes the opposite of seriousness. Thus, the words of the Quran and its warnings about the Day of Accounting are not to be taken lightly, nor are these words weak and without power or effect.
EXPOSITION
In this verse, God is first of all warning the disbelievers not to be foolish and think of the Quran as jest, and also the believers not to merely hear the words and listen to them, but also to make sure that the words are not weak and without power, but that they should have an effect and spark positive action within them.
[1] Jawhari, 5/1850.