This poem is about death, love, and the circle of life…no big deal, right? Uh, okay, fine. Those are some pretty heavy themes. But let's just break it down for you.
"Piedra de sol" is about a person trying very hard to remember someone he's forgotten. He can't remember her face or her name, but it seems like he loved her very much, even though it was a long time ago and he's had a few girlfriends that get mixed up in his memory (cut him some slack, he's an old man). But now he is at the end of life and is hating getting old (this was written pre-Botox) and is doing his best to remember.
Toward the middle of the poem he finally does remember, and it winds up being quite the bummer. Their relationship happened during the Spanish Civil War, in Madrid, and she seems like a really nice girl, one whom he compares to lots of famous mythological and literary women, drowned a long time ago. Yep, big bummer.
The cool part, though, is, after all the sadness, the speaker gets into this really Zen meditation state where he becomes one with the universe, and that's when he figures out that he can be with her. The end of the poem, after all the talk about death, is about rebirth, and ends up with the exact same lines that it starts with, to take us back to the beginning.