Two short lines hold a small, quiet moment: someone is riding the late bus home, leaving behind a place, and possibly a person, that has been loud and demanding. The title's pun, "Bus-y," plays on the word "busy" and on the bus itself, and the poem repeats the idea when it calls the din "busy." Being busy and being on a bus become almost the same thing.
The phrase "up and off" suggests getting on the move, stepping away from something. The word "her" is ambiguous. It might be the speaker's own busyness, a woman's busy noise, or a busy place the speaker is leaving. The poem doesn't say, and that openness is part of its charm. What comes through is a feeling of relief, or at least release. "Again" suggests this is routine, a regular escape. "Late" adds tiredness, with the day winding down. The mood is calm, a little weary, and quietly content as the noise falls away behind the speaker.