The speaker stares into a fire and sees something much bigger than flames: a whole city made of light. Once the image takes hold, it darkens. Burials seem to happen in it "every instant," and the word "plague" suggests a place being emptied out, over and over, as logs collapse and glow and fade.
The "fleet hands rising" could be flickering flames reaching upward, quick and almost human, like people grasping or signaling from below. The last lines shift to a figure "as if from a man without name," someone anonymous who is "not concerned about sleep." Judging by the title, this may be the fire tender himself, or the speaker, keeping watch while others rest.
The mood is quiet, hypnotic, and a little eerie. The speaker doesn't seem frightened. He watches calmly, finding beauty and death in the same burning scene, and the poem leaves us in that late-night stillness.