Three short lines, and the whole scene fits inside them: a tire hanging from a rope, moving gently back and forth. The poem is built around a simple idea, that the tire and the string work together. Rubber is heavy and the string is thin, yet one holds up the other, and the pairing feels almost musical. That's what the word "harmony" suggests.
The speaker doesn't tell us about their own feelings directly. Instead, the calm, watchful tone suggests someone who finds quiet pleasure in noticing a small thing. The last line, "Swinging back and forth," lets the rhythm of the words copy the motion of the swing, so we can almost feel it drifting along.
The mood is peaceful and a little dreamy, like watching something move on a slow afternoon. The poem also hints that ordinary objects can have a kind of beauty if we stop long enough to look.