The speaker looks back with a rueful smile at a mistaken idea. "Fool that I was," they say, they once thought they understood what the word "heart" means. The poem then offers a string of guesses that don't sit together comfortably. Heart is "a raw answer to my mind," something blunt and unprocessed, and then "a cook's shop," a homely, busy place where things are prepared. Next it becomes "an infinite field," and even "a kind of void." The meaning keeps sliding, which seems to be the point: the word won't hold still.
The ending is the strangest part. Into that emptiness the poem places only "a pair / of black horses." We aren't told what they mean, and the poem leaves them unexplained. They give the void a dark, quiet energy, as if something powerful is moving through the space. The mood is wry, humble and a little eerie. The speaker admits they don't have the answer, and the poem seems comfortable leaving the heart as a mystery.