The poem is a single line, and it makes a bold claim: of all the music in a person's life, what counts is what they listen to at night. It doesn't explain why, which leaves us to fill in the reasons ourselves.
Night strips away the noise of the day. There are no distractions, no one watching, and no need to perform. Whatever plays in those hours is heard more honestly, and it tends to meet us when we're lonely, restless, or thinking too much. The speaker seems to believe that this is where music does its real work, shaping how we feel when we're most unguarded.
The tone is quiet and confiding, almost like a secret passed along in conversation. The mood is intimate and a little wistful. The title, "Untitled part," hints that this may be a fragment of something larger, which adds to the sense of a thought caught in passing, brief but true.