The narrator describes the reaper's song as a "melancholy strain" and "plaintive." While he cannot distinguish the words of her song, they sound sad, as if she is singing of loss or pain. He likens her singing favorably to the songs of the nightingale and the cuckoo, both birds that often sing alone. The nightingale is known for its beautiful song, but here, the narrator says the solitary reaper's song is more arresting than even that.
By taking a common, laboring woman and comparing her to a bird, a creature of nature, Wordsworth is depicting her as a thing of beauty in harmony with her environment. This is reinforced by the fact that narrator cannot make out the words: the beauty of the song transcends human language. Her song is sad, but because it is melancholic it is tinged with a bittersweet beauty that touches and thrills the listener's soul.