The speaker says they learn something from timber, from wood and trees, and what they learn is modest: "a thin plain thought." It isn't a grand lesson. It's simple enough that it might slip away, so the speaker hopes to remember it if it ever comes up again "in a true place." The thought itself is to "stand still and do nothing," which is exactly what a tree does.
The poem ends on a small, homely image: a little wine "like rain for supper." Wine and rain connect the speaker's meal to the natural world, as if even supper is something the forest might provide.
The mood is quiet, humble, and content. Nothing is rushed or forced. The speaker doesn't claim wisdom, only a gentle willingness to be still and take what little is given. The short lines and plain words add to that calm, leaving plenty of silence around each idea.