In just a few words, the speaker says that a red line took away their home. The phrase "red-lined," from the title, may point to redlining, the practice of marking certain neighborhoods as off-limits for loans or insurance, though the poem never says so outright. Read that way, a line drawn on a map, an impersonal and bureaucratic act, has a very personal cost.
The second stanza turns somewhere unexpected. Instead of anger or an explanation, we get "all the evening music / great as a prayer." The speaker seems to be remembering what the home held: the sounds of an evening, maybe voices, birdsong, or instruments, and the sense of something almost sacred in them. Judging by the wording, the loss isn't only about walls and a roof but about a whole way of living.
The mood is quiet, sorrowful, and restrained. The speaker doesn't shout. The plain statement of loss, followed by tender memory, makes the grief feel deeper.