The speaker describes turning inward, away from the world, toward a private place he pictures as an underground lake. He calls it "the Beloved Lake," and the more time he spends there, the more he explores it and the more he knows it. Everything else seems to fade. He admits a cost, too: he is "blinder" from spending so much time there, as if staring into the dark has dulled his sight for anything outside.
The mood is absorbed, quiet, and a little uneasy. He says his work comes out of this "personal preserve" and is better known to him than other ways of working, while outside places give less and only distract. Yet the poem seems to undercut its own confidence. The syntax stumbles and loops, and the last line, "keeping my attentions as clear," simply stops, unfinished. Judging by that ending, the focus he wants may be slipping even as he tries to describe it.