The poem opens in what's implied to be a military hospital, and focuses on men suffering from shell-shock (which is now referred to as post-traumatic stress disorder, or PTSD). The poem starts with a series of rhetorical questions that ask the reader to consider both who these men are and why they sit “in twilight.” The speaker asks why (“wherefore”) these men have been reduced to “shadows” of their former selves, and now spend their days in pain, rocking back and forth and drooling from their open mouths. The grotesque imagery here gives the reader a shockingly vivid sense of the scene inside the hospital.
As literal questions, the answers are obvious: these are men trying to recover from shocking war experiences, and they are in the hospital as part of their convalescence. But these questions also gesture towards the fact that—as the rest of the poem makes clear—these men no longer seem human; language about "shadows" makes it seem as though it's they have become the suffering ghosts of their former selves. These questions thus speak more widely to the sense of the war as an incredible, unjustifiable waste of human life. That is, the speaker isn't really asking the immediate why they're sitting there in the military hospital, but why the war that led them there happened in the first place.
Diving into more specifics of those questions, the mention of “purgatorial shadows” is likely an allusion to Dante’s Inferno. There, and elsewhere in literature, the souls of the dead are known as “shades,” the implication being that they are no longer physically present in the world, and instead are mere shadows—like the negative of a photograph—cast by their former waking lives.
What’s interesting here is how the speaker conflates this with purgatory, which is an in-between place that is neither heaven nor hell. The idea is that, though the men are almost as good as dead in the sense that they can no longer function, the fact that they are still alive constitutes a kind of ongoing, undecided state. That is, they are caught between the poles of the living and the dead, not quite one or the other. Indeed, the poem subtly implies that either option—being fully alive or being fully dead—would be better than being barely alive.
Notice how the language here actually sounds gruesome too—it’s intentionally graphic, designed to shock the reader into empathizing with the wounded soldiers. The assonant /aw/ of "jaws that slob" evokes that loose-mouthed slobber, for example. The consonant /k/ and sharp /t/ sounds then make the lines sound almost like they're being spat out (the /s/ sound adds to this effect):
Wherefore rock they, purgatorial shadows,
Drooping tongues from jaws that slob their relish,
Baring teeth that leer like skulls' tongues wicked?
Stroke on stroke of pain, [...]
The diacope of "Stroke on stroke" underscores the intensity of the soldiers' pain, playing on the way that “stroke” usually relates to something gentle. The men’s pain is constant and real, but it’s also a kind of phantom, something without physicality. "Stroke" can also refer to time, as in the "stroke of the clock." Immediately, then, the reader is given a powerful impression of ongoing, day-to-day, minute-to-minute torture.