Rainy Day describes an ordinary wet afternoon in a few quick, simple lines. The speaker notes that it rained, that the raindrops fell on their head, and that mud from the road stuck to their new rain boots. Nothing dramatic happens, and that is part of the charm: the poem pays attention to small, everyday moments.
The speaker's feelings are never stated outright, but the details hint at them. The word "little" and the playful "dripped-dropped" make the rain sound light and almost friendly, as if the speaker is amused rather than bothered. The mud is a different matter. "Stuck and clung" slows the rhythm down and suggests a slight annoyance, especially since the boots are "new" and now getting dirty. The mood is quiet, gentle and a bit humorous, the kind of feeling you get when a small mishap spoils a fresh start but doesn't ruin the day.