Two short lines carry a whole quarrel and its ending. The title tells us an argument has happened, and the poem skips the shouting to give us the quiet afterward: by evening, the speaker and someone else have come to agreement.
What makes it work is the comparison. A barn owl returning to a barn doesn't arrive with fanfare. It glides in at dusk, silent, to a place it already knows. Agreement, the poem suggests, is like that: not a victory or a bargain struck, but a settling, a return to somewhere familiar and shared. The word "evening" helps, since it brings the sense of the day winding down and tempers cooling.
The speaker seems relieved and calm rather than triumphant. There's tenderness in the image, too, because a barn is shelter. The mood is hushed, gentle, and a little wistful, as if the speaker is quietly grateful that the argument ended where it did.