A short moment: rain begins to fall, and an old swan looks afraid. The poem doesn't say what the swan is afraid of. Instead it offers a comparison, that the bird seems "at a loss / how to sing." Swans are traditionally tied to the idea of a final, beautiful song before death, so an old swan that can't find its song carries a quiet sense of ending, or of something failing just when it matters most.
The speaker mostly watches, but the choice of words shows how they feel. "Fearful" and "at a loss" are human ways of reading a bird's face, so the speaker may be seeing their own worry in the swan. The mood is hushed and uneasy, a little sad. The rain gives a gray, heavy backdrop, and the short lines and gaps between them leave the same kind of silence the swan seems caught in. We're left with a small, still picture of age, uncertainty, and a voice that won't come.