In this poem, the author describes the "music" that the movement of the black girl brings to our ears. He talks about the way in which the girl's playing makes her braids move, and he describes this musicality by using words such as "symphony","crescendo", and "movement." These words are employed as imagery, and their effect is that they create an image in the mind of the reader. This image contributes to the meaning of the poem by portraying the vivacity and cheerfulness of the girl.
I think the answer is the reason for his absence is quite simple to understand.
