A girl lives in a cave, and when she thinks no one is watching, she tidies up, lights her lamps, and dances a few steps. Then she stops and goes quiet, afraid someone might be looking. The poem is about that small, private moment of joy and how easily it gets shut down by self-consciousness.
The mood is gentle but a little uneasy. The night seems cautious around her melody, and the "howls & giggles" sit side by side in a strange, eerie way, as if playfulness and fear share the same space. The speaker seems tender toward her, even admiring. The line about the world and the girl each wondering at the other suggests that her hiding isn't because she lacks anything worth seeing. The closing line, "the prettiest things are always in hiding," makes the cave feel less like a prison and more like a shelter for something delicate. Judging by the imagery, the cave may stand for shyness or privacy.