The speaker picks up a small brown rock from the worn ground where he walks, near chalked hopscotch squares, and starts to think about what it means to know something. He admits he can't say how the rock was made or how it got here. Time, pressure, a sudden break: he knows none of it. That honesty becomes the poem's main move. He sets himself against people who claim too much, like a French academic who says he knows what makes a whole people different, or a taco chain boasting about its expertise in bland burritos. Both are played for gentle mockery.
Instead, the speaker settles for modest affection. He likes the rock and whatever it is expert in, and the last line, "It's this rock, until it's not," accepts that it will eventually change or break. The mood is wry, humble and quietly tender. The opening nod to Erasmus and Folly adds a joke: he won't praise foolishness, yet he ends up praising something small and unknowable.