The speaker addresses someone restless and hard to hold onto, a "vagabond" with flickering eyes. The person is pictured dancing in a storm, drifting so far into the wind that they seem to be slipping out of reach. The speaker doesn't scold them. The tone feels closer to wonder mixed with worry, since being "spirited and untamed" is clearly part of what makes this person captivating.
The poem is short and ends abruptly, and that works in its favor. The last line, "When the brilliant day darkened," arrives without a full stop to the thought, so the sentence hangs there like something unfinished. We never learn exactly what happened. The word "almost" adds to this uncertainty: the person was almost lost, or almost tamed, and the poem leaves that open. The mood is wistful and a little uneasy, a bright, wild energy shadowed by the sense that it could vanish. Judging by the imagery, the speaker may be remembering someone who has already gone.