In just three lines, this tiny poem puts us inside a quiet, funny, tender moment. Someone, the speaker's "girl," is combing the speaker's head. The speaker then imagines how they look through her eyes: "to whom I am an ape." It's a playful joke, and it also reads as affectionate. An ape being groomed is a creature being cared for, the way primates pick through each other's fur as a sign of closeness.
The last line, "I rain down wool," shifts the image. Hair becomes wool, and the speaker becomes something half animal, half sheep, shedding softly under the comb. The word "rain" makes the falling hair gentle and almost pretty rather than messy.
The speaker seems relaxed, amused at themselves, and happy to be looked after. The mood is warm, intimate and lightly self-mocking. Nothing dramatic happens, but the poem finds something a little wild and a lot loving in an ordinary domestic act.