The speaker asks us to remember a rock we once placed in a river as a child. Back then it felt enormous, as big as a bouncy ball we could barely wrap our arms around, and we guarded it fiercely, telling the whole neighborhood that no one could ever move it. The rock stands for the things that seem huge when we're young: a favorite place, a belief, a small project that mattered more than anything.
As we grow, the rock seems to shrink, and its hold on us loosens. The speaker doesn't treat this as sad, though. They say it's "still good," and imagine the rock wearing away until it joins the ocean or a lake, "ultimately a better place." The tone is gentle, nostalgic and calm. The ending offers a quiet comfort: the rock will still be loved and remembered, and letting go is natural, because "something new needs to arise."