Four short lines and a single word set the whole scene: a lake, a bird passing over it, and a person who stays behind. The bird is "unnamed," so we never learn what kind it is. It simply skims the surface, quick and light, touching the water without disturbing anything. It belongs to the moment and moves on.
The second part turns to "you." Whoever is being addressed lingers, and the poem calls that an "afterthought." The word is ambiguous. The person might feel overlooked or secondary, left at the edge of things while the bird passes through. Or the speaker might be describing someone who stays quietly, almost unnoticed, as the scene fades.
The speaker's feeling is never stated outright, which makes the mood gentle and wistful. The short lines and the white space around them create stillness, like a lake after something has crossed it. What remains is a faint sense of loneliness, and of being slightly out of step with the movement around you.