Allah informs that the criminals used to laugh at the believers in the worldly life. In other words, they would mock them and despise them. Whenever they would pass by the believers, they would wink at each other about them, meaning in contempt of them.
(And when they returned to their own people, they would return jesting.) meaning, when these criminals turn back, or return to their homes, they go back pleased. This means that whatever they request, they find it. Yet, with this, they still are not grateful for Allah's favor upon them. Rather they busy themselves with despising and envying the believers.
(And when they saw them, they said: "Verily, these have indeed gone astray!") meaning, `because they are upon a religion other than their own religion.' Allah then says,
(But they were not sent as watchers over them.) meaning, these criminals have not been sent as guardians over the deeds and statements of these believers. These wrongdoers have not been made responsible for them. So, why are they so concerned with them, and why have they made them the focus of their attention This is as Allah says,
(He (Allah) will say: "Remain you in it with ignominy! And speak you not to Me!" Verily there was a party of My servants, who used to say: "Our Lord! We believe, so forgive us, and have mercy on us, for You are the Best of all who show mercy!" But you took them for a laughing stock, so much so that they made you forget My remembrance while you used to laugh at them! Verily, I have rewarded them this Day for their patience: they are indeed the ones that are successful.) (23:108-111) Thus, Allah says here,
(But this Day) meaning, the Day of Judgement.
(those who believe will laugh at the disbelievers) meaning, as retribution for how those people laughed at them.
(On thrones, looking.) meaning, looking at Allah as reward for bearing the false claims against them that they were misguided. They were not misguided at all. Rather they were the close Awliya' of Allah, who will be looking at their Lord in the place of His honor. Concerning Allah's statement,
(Are not the disbelievers paid for what they used to do) meaning, `will the disbelievers be recompensed for their mockery and belittlement against the believers, or not' This means that they surely will be paid in full, completely and perfectly (for their behavior). This is the end of the Tafsir of Surat Al-Mutaffifin, and all praise and thanks are due to Allah.