Walking, for the speaker, isn't exercise or a way to get somewhere. It's a way of staying curious about "the unself," a striking word for whatever lies outside the speaker's own thoughts and habits. The idea seems to be that ordinary life can shrink our attention down to ourselves, and that a walk loosens that grip.
The second stanza pushes further: "venture further / to some obscure place." The walk becomes a small expedition, away from the familiar. What the speaker finds there is surprising and lovely: "a piece / of green and busy night." Night is usually thought of as dark and still, but here it is green and full of activity, which suggests plants, insects, and animal life going on unseen.
The mood is quiet, attentive, and quietly delighted. The short lines and plain wording mimic a slow, careful pace, and the final image feels like a small reward for paying attention.