In just three short lines, this haiku-style poem gives us a small domestic scene: someone busy cooking in a back kitchen, and clearly in a bad mood. The phrase "quite out of humor" is a slightly old-fashioned way of saying irritable or put out. We aren't told why. Maybe the meal isn't going well, maybe the work is tiring, or maybe the day has simply worn the cook down.
Then the last line shifts our attention outward: "evening rain." It could be a gentle contrast, with the soft rain falling calmly while the cook fumes inside. It might also match the cook's mood, gray and heavy at the end of the day. The poem leaves that open, and I think that's its charm.
The mood is quiet, a little grumpy, and oddly cozy. We can almost hear the clatter of pans and the patter of rain outside, and we recognize the feeling of being stuck in a task when the day is winding down.