We know by now that a tree isn't just a tree in this poem. Well—it is and it isn't. Ashbery seems to be using both ideas: one that gets at the idea that we should just dig things for what they are and another that tells us that "some trees" represent a bigger, human idea. They stand alone and yet they're part of a bigger community, just like people. They've also got something to say to the speaker, so the next time you find yourself chilling in the park, have another look around to appreciate those awesome trees.
Title: The title says it all: some trees. We can make a lot out of them, or nothing at all. And that's the point.
Lines 1-2: And they are "amazing," according to the speaker. After all, they get him thinking about all sorts of things just because they're there. And they have "neighbors" just like people. They stand alone but not entirely alone. They're "joined" whether they want to be or not.
Lines 7-8: Here's when we get what the trees are trying to tell the speaker. What exactly that is, we're not so sure, but again, that's kind of the point. We get the full idea of the trees being an extended metaphor for relationships. The "you and I" are nestled nice and cozy with the trees. The enjambment between lines furthers the metaphor.
Lines 10-11: Then we get what those trees are trying to say: "That their merely being there / Means something." Again, there's no word as to what that "something" is, but the speaker's not pretending to be a know-it-all. He's just trying to appreciate the trees, and his relationship, for what they are.