Nobody appears in these three lines. There is no child, no yard, no summer afternoon, only a piece of rubber, a string and a movement. What the poem is about is a tire swing seen as a partnership, and its central idea is that harmony does not mean sameness. It means two unlike things working together. The rubber is soft and heavy, the string thin and plain, and the swing exists only because each does its own job.
The first line sets this up in an unusual way: "Rubber's harmony." The possessive makes harmony something the rubber owns, almost a quality of its character. Yet the next line immediately says the rubber cannot have it alone: the harmony is "With the string that holds it up." The title does the work of naming the object, so the poem never needs to say "tire." It can say "rubber" instead, shifting our attention from the thing to its material, its give and weight. It is a small choice, but it makes us think about what a tire swing is made of rather than what it looks like.
"Harmony" is also a word from music, and the poem seems to know it. Once we hear it, "string" can sound like an instrument's string, and "swinging back and forth" becomes something like the slow vibration of a plucked note. The poem never says anything about sound. Judging by the imagery, though, it invites us to picture the swing as a quiet instrument, with the rubber as the weight and the string as the thing that makes a steady, repeating motion possible.
The grammar adds to this effect. The poem has only one conventional verb, "holds," and it is a verb of support, not action. The first line is a bare noun phrase, which feels still. The last line is a participle, "Swinging," with no subject attached, so nothing in the sentence ever owns the motion or brings it to an end. The text as given has no punctuation, so there is no full stop either. The swing is left moving.
The form is compact. The lines count 5, 7 and 5 syllables, the pattern of a haiku, and there is no rhyme. The longest line sits in the middle, and it is the line about holding: "With the string that holds it up." The form quietly copies the idea, since the longer middle line supports the two shorter ones around it, just as the string supports the rubber. The poem does not follow the traditional haiku habit of naming a season, and its attention stays on a single mechanical image.
Sound does some of the work that rhyme usually would. "Rubber's" bounces on its doubled b, a fitting sound for the material. "String" and "Swinging" share both their opening consonant sweep and their -ing ending, which links the support and the motion in the ear before we think about it in the mind. "Back and forth" is a perfectly balanced phrase, going out and returning like the thing it describes, and "forth" leaves the last beat pointing outward, as if the swing has not yet turned around.
Part of what makes the poem worth reading is its restraint. It could have talked about childhood or freedom or nostalgia, and it chooses not to. A reader may supply a child on the swing, but the poem leaves the seat empty, so the swing seems to move on its own, held by something small. That makes its quiet paradox easier to see. The swing is free to move only because it is tied, and the string, thin as it is, makes all the motion possible.
For students, the poem shows how much a short text can do with what it leaves out. With no characters and no story, we are left to look closely at word choice, line length, grammar and sound, and each of them turns out to be carrying the same idea. The poem's claim is gentle but real: harmony comes from the balance between holding and moving, and in the right arrangement, a heavy thing and a slight thing can make something that looks effortless.