The title of the poem isn't exactly cryptic. This poem describes a painting that shows a bunch of peasants dancing, so it's called—wait for it—"The Dance." Well, maybe it's not quite that simple. What's interesting to us about the title is that it highlights the sense of motion that the speaker feels from the painting. Though the figures of the canvas are forever frozen, the speaker is responding to the feel of movement expertly captured by the painter, Brueghel. This sense of movement is also embedded in the swirling rhythms and sounds of the poem, itself, making the choice of the title pretty spot-on.
The title is doubly appropriate when you consider that Williams, in his poem, is in a sense pairing up with the painting. Any writing about the visual arts is known ekphrasis. It's a specialized form in which the writer is trying to capture in language what the artist has done with line and color in a visual way. Williams here is really engaged in a kind of dance with Brueghel, trying to wrest the artist's effects out of the painting, then twirl and render it in a poem.
(Side note: Brueghel is actually known for having been obsessed with capturing movement in his paintings. We're guessing he'd be pleased that Williams responded to that very thing in his work.)