The poem imagines what happens to people when they step into a field: they stop being quite themselves. The speaker uses the word "prisoner," but the captivity isn't harsh. Instead, the landscape becomes a "green page," a place to retreat to, as if the grass were something we could read or write on. The title, "Born-again naturalists," hints at a sudden, almost religious conversion, a fresh attention to the natural world.
The last image is the strangest. The ground we touch is "abstract," as though it's more an idea than solid earth, and we stand "like oaks engaged / with the night." Oaks seem patient and rooted, quietly taking part in the dark rather than fearing it. The mood is calm, dreamy, and a little mysterious. The speaker seems to feel that being in nature loosens ordinary identity, and that this loss is a kind of relief, even a way of belonging.