The poem is a tiny contrast between being with others and being alone. In the first half, the speaker remembers a "round house" where everyone was "merry and good-natured." The word "round" suggests a shared, enclosed space with no corners, where people sit close and nobody is left out. The mood is warm and easy.
Then the poem turns on a single word: "alone." The cheerful company vanishes, and what remains is a "melancholy violin" that calls the speaker to supper. It's an odd, striking image. The violin could be a real instrument, or it might be a way of describing how a lonely house sounds, or how the speaker hears a summons to the table when no one else is there. The poem doesn't say which.
What the speaker feels is quiet sadness, but it's understated. Nothing dramatic happens. The poem simply lets the gap between the lively "together" and the thin, sorrowful sound of being alone speak for itself.