A speaker stands outside before dawn and hears voices drifting down through the fog. The poem never says whose they are, but the images that follow suggest migrating birds passing overhead: "fast moving shadows / against the stars" and a "far-off tundra" fit that reading, though the poem leaves it as a guess. The sound is compared to the year's first snow crystals, which makes it seem delicate, rare and a little unreal.
The speaker can't see anything, so the imagination takes over. Hearing the voices, they picture the cold north, where night lasts for months. Then the poem comes back to the ordinary: a porch rail, a creek swollen by rain. The final lines show how deeply the moment stayed with the speaker. Two weeks later, they are still hearing "the last treble notes."
The mood is quiet and hushed, full of wonder. It suggests that a brief, half-seen event can linger far longer than it lasted.