Watching people dance in the street, the speaker sees something older than the scene itself. The opening lines say humans haven't traveled as far from our beginnings as we like to think. Our brains, "those soft machines," still sit inside "caves of bones," meaning our skulls. The image quietly pictures early humans sheltering in caves, and the line "There are bears among the stones" suggests the old dangers are still lurking, at least in memory or instinct.
Then the poem turns to the dancers: a woman twirling, a man in a green shirt, flirting girls, elderly people with their small dogs, children in "wild cavort." The speaker seems to be saying that dancing is one of those deep, shared habits that connects us to our distant past. The mood is a mix of wonder and unease, with the warm, ordinary scene set against a hint of something primal. The speaker doesn't judge anyone, but watches with tenderness and a sense of awe.