What does « Burning thirst » mean in the Quran?

It was not [proper] for the people of Madinah and those surrounding them of the bedouins that they remain behind after [the departure of] the Messenger of Allah or that they prefer themselves over his self. That is because they are not afflicted by thirst or fatigue or hunger in the cause of Allah, nor do they tread on any ground that enrages the disbelievers, nor do they inflict upon an enemy any infliction but that is registered for them as a righteous deed. Indeed, Allah does not allow to be lost the reward of the doers of good.

And indeed, you will not be thirsty therein or be hot from the sun."

But those who disbelieved - their deeds are like a mirage in a lowland which a thirsty one thinks is water until, when he comes to it, he finds it is nothing but finds Allah before Him, and He will pay him in full his due; and Allah is swift in account.