The title tells us this is an elegy, a poem of mourning, and the rest slowly shows why. The speaker watches a single leaf fall, and the scene is set in a kind of in-between: between rain and snow, summer and winter, here and there, now and then. Nothing is settled. The season is turning, and the speaker seems caught in the same uncertain space, neither past the loss nor free of it.
The repeated line, "a lone leaf was falling," works like a slow heartbeat or a held breath. Then the ending turns everything around: "it was you / alone." The leaf was never just a leaf. It stands in for a person who has died, and the word "alone" lands twice over, describing the leaf, the one who is gone, and perhaps the speaker left behind. The mood is quiet, still, and sorrowful. The speaker doesn't cry out. Grief shows up in the small, plain act of watching something fall.