The speaker is in the grip of a secret crush, or at least a feeling they are trying hard to keep down. They call it "vile," which suggests they are ashamed of it, or afraid of what it might mean. Yet the poem's second line shows how hard it is to hold back: just by being near this person, the speaker's heart "beats and it skips."
The scene is small and quiet. The speaker stands "by the sides," watching from the edge rather than stepping forward, and that placement says a lot about their hesitation, which the title also names. Then comes the giveaway in the last line: a smile creeps onto their lips, and they can't stop it. The poem's mood is tender and a little nervous, a mix of shyness and delight. The speaker wants to hide what they feel, but their own face keeps telling the truth.