"Sow" is a poem about just what it sounds like: an adult female pig. Lines 1-6 frame this sow as a formidable and mysterious creature. She belongs to the speaker's "neighbor," a farmer, and is a source of local amazement: "God knows how [he] managed to bread / His great sow," the speaker marvels.
The word "great" here primarily means large, but in the context of the poem as a whole, it could refer to a more intangible kind of greatness as well. The plural speaker ("our") doesn't identify themselves, but clearly represents two or more people who live near the farmer. (The poem was inspired by a real-life visit Plath and her husband, fellow writer Ted Hughes, took to a farm in rural England.)
Whatever method the farmer used to breed such a large animal, he keeps it a "shrewd secret." In fact, he won't show either the method or the pig to most of his neighbors. He keeps the sow "impounded," or locked away, "from public stare, / Prize ribbon and pig show." That is, he keeps her hidden from curious onlookers and won't enter her in any local livestock competitions (the kind where a pig might win a prize). It seems this animal attracts so much fascination and speculation that she's taken on the quality of a divine mystery: only "God" seems to know the full truth about her.
These opening stanzas establish the poem's form: tercets (three-line stanzas) with occasional rhyme (including slant rhymes, such as "breed"/"hid" and "sow"/"show") but no consistent rhyme scheme or meter. The poem seems a little resistant, in fact, to strict formal rules—like a pig that's not quite happy in its pen.
The lines are densely packed with alliteration ("sow"/"secret"/"same"/"sow"/"stare"; "public"/"Prize"/"pig"), as well as assonance and internal rhyme ("how"/"sow," "same way," "sow"/"impounded," "ribbon"/"pig"). These sound effects give the language a slow, weighty, robust quality that fits the poem's subject.