"Meditation" could make you think of religion. Hass is really interested in Eastern religions like Hinduism and Buddhism, so that’s part of what he’s up to. But, "meditation" has another meaning, which is a kind of philosophical discussion where you allow yourself to think hard and long about some idea. One of the most famous Western philosophical texts of all time is called "Meditations on First Philosophy," written by the Frenchman Rene Descartes.
Even though Hass doesn’t consider himself a philosopher, the poem discusses heavy philosophical subjects, like the relationship between language and truth and the nature of reality. It’s probably best to think of the title as a poetic response to philosophy, one which incorporates a kind of Eastern "meditation" as well.
"Lagunitas" is a beautiful, rural town in Northern California, where Hass lived for much of his life. It’s a great place to go hiking: lakes, forests, and green grass all around. And, yes, it’s the kind of place where you might see blackberries growing in the wild. We can imagine that the poet is writing (or, at least, composing in his head) these lines as he walks around looking at the natural scenery. So, when he talks about "that black birch" in line 6, think of him looking at an actual tree somewhere near Lagunitas. Like the woodpecker, the tree, and the blackberries, Lagunitas is a very specific and particular thing, and not just a "general idea" (line 4).