Three short lines carry a quiet confession. The speaker says they "mar" their day by talking, spoiling something that would have been better left untouched. The title, "Trappist," points to the monastic idea of silence, so the poem reads as a small lapse from that ideal: the speaker wishes they had said less.
The last line is the surprising one. The talk is "busy as a night ear," and that comparison is open to a few readings. At night, with everything else quiet, the ear strains and picks up every small sound, so it is restless and alert. The speaker's chatter may be just as fidgety, filling the air with noise it doesn't need. Judging by the imagery, there's also a hint that talking, like listening in the dark, can't switch itself off.
The mood is hushed and rueful, with a touch of dry humor. It feels like a private note to self, spoken softly.