ٱلنَّارِ ذَاتِ ٱلْوَقُودِ
This verse is the appositive substitute (the explanatory term) for the word ukhdūd in the preceding verse. That is: "the trench — that is, that fiery trench".
What the word order does is this: the fourth verse said the pit, the fifth said what was inside it. The camera first shows the hollow, then looks into it.
Root: و-ق-د. To catch fire, to burn. Waqada'l-nāru — the fire caught. Īqād — kindling. Mawqid — the hearth.
وَقُود (waqūd, in the reading with fatḥa) — fuel, that is, the matter that feeds the fire. The Qurʾān uses this word in a striking place:
"…guard yourselves against the fire whose fuel is people and stones." (al-Baqara 2/24) "…protect them from a fire whose fuel is people and stones." (al-Taḥrīm 66/6)
A note on variant readings: the reading وُقُود (wuqūd, with ḍamma) is also transmitted in the sources, and then the sense becomes "the burning itself". I give no firm ascription as to which imām of the readings held this reading. In point of meaning the difference is small.
Dhāti'l-waqūd — "holder of the fuel". The verse does not say "a great fire" or "a fierce fire"; it qualifies the fire by what feeds it.
This gives a piece of information about the scene: the fire is not burning of itself. Someone is feeding it. Fuel is carried, thrown on, the fire is kept going.
What is being related, then, is not a momentary outburst of rage; it is a sustained operation. The next verse will complete this: they were sitting. Sitting is the sign of a work that has gone on a long time.
And when the usage in al-Baqara 2/24 and al-Taḥrīm 66/6 is called to mind, a correspondence arises: in this world they lit a fire whose fuel was human beings; in the hereafter there is a fire whose fuel is human beings. The sūra does not say this; the word says it.