The speaker feels watched, and the poem is about what that does to a person. It opens with a small, almost beautiful image: a fire burning inside a glass house. Everything is visible, nothing can be hidden, and even something as private as a fire becomes a display. Then the view widens. "They" have built a whole town in glass, so the exposure isn't just personal but shared by everyone living there.
The question in the middle, "who appointed them / to see and to find us," is the sharpest moment. The speaker sounds uneasy and a little angry, challenging the authority of the watchers without expecting an answer. The final lines turn inward. The body becomes a flame that provokes itself, which suggests that being constantly observed can make us anxious or restless, even when no one is acting on us directly. The mood is tense, cool and brittle, much like the glass itself.