A small bird has found its way into a library, and the speaker watches it with a mix of tenderness and quiet envy. The wren seems to carry "a luster of lost time", as if she brings in something bright from a world of hours that have slipped away. The poem doesn't explain what that means, and that openness is part of its charm.
The middle lines turn strange and funny. The bird is "as foolish as a lord", yet also tied to "the beggar's" condition, so she seems both grand and poor at once. The poem leaves that comparison loosely connected, and the reader has to fill in the gap. The ending is the most moving part: "nowhere else to read / and to be." Reading and simply existing become the same need. The speaker may be talking about the wren, but the words could easily describe the speaker too. The mood is gentle, wistful, and a little playful, like a small moment of shelter that the poem is careful not to disturb.