A speaker builds a window to shut out the cold, a plain, practical job. Then a wren makes her home there, and the plan quietly changes. The window was meant to protect the speaker, but now it shelters someone else too.
The second half moves indoors. All morning the speaker searches through books for "a share of quiet," as if looking for the same kind of shelter the bird found. The last lines are the most interesting: the quiet turns out to be "more than I thought I should / open up to." That phrasing suggests the speaker feels a little uneasy, even guilty, about letting so much stillness or openness in.
The mood is calm and gentle, with a slight hesitation underneath. The title, "Infrastructure," makes the poem feel like it's about more than a window: the structures we build to protect ourselves can end up letting something unexpected in.