The speaker is a "shut-in," someone closed off in a room, and the poem imagines the obvious escape: smashing a window. But the first line already doubts it. Breaking the glass, the speaker says, "will enlighten no one." Opening up the space won't bring clarity.
The next lines explain why. The quiet would be lost, and the boundary between inside and outside would vanish: "in will be out." That seems to be a loss rather than a freedom. The final stanza is the sharpest, and a little wry. New sights won't settle the mind; they'll just give it "more thoughts" to churn through.
The speaker comes across as wary, maybe a bit weary, and oddly protective of their enclosure. The mood is still and contained, with a dry humor underneath. Judging by the wording, the room is less a prison than a refuge from mental noise, and the poem quietly questions the idea that more openness always means more understanding.