Before the poem begins, Sassoon indicates that this was written and/or is set along the "Hindenburg Line" in April of 1917. This refers to a long German defense system in western Europe during World War I.
The setting established, the poem drops the reader right in the middle of the action: a soldier is clumsily making his way through a dark tunnel. The lack of specifics—who this man is, or the nature of his mission—might leave the reader a bit disoriented, much like the soldier himself! Indeed, it's not clear if the soldier knows exactly what he's supposed to be doing. He feels his way through the underground, gripping the walls and inching along "step by step," the diacope of the phrase giving the poem a slow, tentative feel.
His torch (a.k.a flashlight) illuminates patches of the stuffy darkness. The speaker personifies that torch, referring to it as a kind of "prying" eye "glar[ing]" into the tunnel:
He winked his prying torch with patching glare
From side to side, and sniffed the unwholesome air.
The torch seems to be the only sign of life down there, and that "prying [...] glare" suggests that it's uncovering things that shouldn't really be (or don't want to be) seen. The enjambment between these lines also makes it feel like the poem itself is probing the darkness, trying to find its way.
Notice, too, how the diacope of "side to side" mirrors "step by step" from line 1. Again, this makes the soldier's actions seem tentative, as though he isn't really sure what to do or which way to go. One thing's clear, though: it stinks down there. Young men have been living there for days, weeks, or even months on end—and, as will be revealed later in the poem, there are also dead and decaying bodies nearby. The poem doesn't need to do too much with its imagery here—just noting that the air is "unwholesome" suggests something horrendous, something deeply wrong, has happened here.