The poem pictures a prison yard, though the title makes us wonder whether the "prison" is literal or a way of describing ordinary life. The yard is "varnished," a glossy, artificial surface, and the speakers "bake" in it, as if they are being slowly cooked. There is no escape from the heat or the sameness.
The speaker says "we," so the feeling is shared. It is weary and cornered, with a flash of anger in the line about offering "violence / to the lip and the window," which suggests lashing out at speech and at the view of a world outside. People are "tormented by bad clothes" and walk to "cages" at night, so even rest feels like confinement.
The final image turns them into fat birds "mining for home." Birds don't mine, so the phrase feels strange and a little sad: a hopeless digging for belonging. The mood is bleak, restless, and quietly pained.