Four lines, no punctuation, no capital letters, and almost no way to paraphrase them without losing something. Jim Leftwich's "Antithesis 17" gives us two short couplets in which grand, airy things are dragged down into the material world, and the strangeness of those collisions is the point. I'll stick to what the text shows, since the poem itself offers little context beyond its title. The number 17 suggests a sequence, but nothing in the four lines tells us more.
The first couplet begins with a lowercase "as," so the poem seems to start mid-thought, as though a comparison has already been under way. It never completes: "as an idea on a errand" has no verb and nothing for the "as" to attach to. An idea is weightless and abstract, yet here it is sent off like a messenger running a small, dull job. The phrase "a errand" is ungrammatical, since standard English wants "an." Whether that is deliberate or an accident of the text, I can't tell from the poem alone, but it roughens the line and makes the reader stumble over the vowels, which suits a poem about things bumping into each other.
The next line stays physical: "snow pasted onto its brother." Snow is soft, cold and temporary, while paste is sticky and deliberate, a word from the world of glue, scissors and collage. Snow stuck to its own brother, presumably more snow, is like joined to like, a redundancy that looks like kinship. That family word matters, because the second couplet turns the same idea into politics.
"class fidelity has been a potato" is the longest line and the only one with a main verb. Fidelity means loyalty, and loyalty to one's class is an abstract, even ideological notion, until the poem calls it a potato. The potato is humble, starchy, and grown underground, and it is the stereotypical food of the poor. The present perfect "has been" makes it sound like a verdict on history: that loyalty was only ever something plain and edible. Read beside the snow and its brother, the line suggests a bond with your own kind, whether you chose it or not.
The last line, "unto the thieves of rust," adds an archaic, almost scriptural "unto." That word makes the potato sound like something handed over or surrendered. The thieves and the rust together also recall, I'd suggest, the New Testament warning about treasure that moth and rust corrupt and thieves steal, though the poem never says so. What it does do is combine a human agent (thieves) with a slow, impersonal process (rust), so that theft becomes decay. If loyalty is a potato given to such thieves, it is being consumed or corroded rather than defended.
The title helps explain the construction. An antithesis sets opposites side by side, and every line here is built from one: idea against errand, soft snow against hard paste, abstract fidelity against a vegetable, and thieves against the rust that steals without hands. There is no end rhyme, but the sound does hold the lines together. The long *o* runs through "snow," "onto," "potato" and "unto," so the two couplets echo across the gap between them, with "onto" and "unto" nearly mirroring each other. Hard consonant clusters in "pasted," "class" and "rust" give the lines a clipped, gritty feel against the soft vowels. The rhythm is uneven, with lines of roughly eight, seven, ten and six syllables, and the shortest line comes last, so the poem ends in a dry, brittle stop.
As for what makes it significant, the poem asks us to read differently. It doesn't tell a story or state an argument. Instead it supplies phrases that grammar allows but common sense resists, and it leaves us to build the connections: snow to brother, brother to class, class to potato, potato to rust. A reader might decide the poem is about loyalty and exploitation, or might decide it is mostly about how language behaves when ideas are forced to run errands. Either answer is possible, and the poem is built to make us notice that we are the ones doing the work of making sense.