The poet notes that smart people have tried to make sense of loss for a long time and, in particular, how language causes us to misunderstand the world. Some people think that talking about specific things in the world takes away from the more general idea of Things, with a capitol "T." In other words, the world of ideas is fuller and richer than the world of real, everyday things, like woodpeckers and blackberries.
People also say that the words we use don’t refer to real things at all, but to things that have been "lost." Sweet, tasty black things may grow on bushes, but the word "blackberry" refers to something that doesn’t exist anymore.
The poet remembers a philosophical conversation about loss with one of his pals. He feels frustrated and realizes that philosophy often just consists of pointless word-games, which seem to deny the most obvious facts of reality. Then, he remembers a woman whom he slept with, and how he is most interested in how she makes him think of long-gone things from his childhood, which has nothing to do with her. She probably feels the same way.
Finally, he decides that, sometimes, real things are just as good as pretty words and fancy ideas. Sometimes, even, the word "blackberry" can be a real thing that’s just as sweet as the fruit itself.