In just three lines, this poem gives us a small, strange scene: a rat walking across tallow, the rendered animal fat that a rendering plant produces. The title tells us where we are, a place where animal remains are processed, and the poem catches a moment "before four o'clock," perhaps early in the morning, when the place is quiet and no one seems to be around.
The last line is the surprise. The tallow is soft, and the rat is "as soft as it." The comparison is ambiguous. It could mean the rat moves softly, without a sound, or that its body seems to blend into the fat, as if the creature and the material were nearly the same thing. The speaker doesn't tell us how they feel, but the observation is calm and close. The mood is hushed, a little eerie, and oddly gentle. A grim setting becomes almost tender, while still reminding us of how life and waste touch each other.