The poet, rambling about in the Lake District of northern England, hears the song a woman reaping in a field by herself. He tells us it is a "melancholy" song, and that no nightingale ever sang a more welcome tune to a weary traveller. He likens the woman to an object of nature as he also compares her song to that of the cuckoo, saying the cuckoo's singing was never so "thrilling" as the reaper's. So we learn that the reaper sings a sad, beautiful, thrilling tune that stops the weary poet in his tracks.
The poet can't make out the words of the woman's song, so he decides to imagine what the subject might be. He wonders if she sings an "unhappy" ballad about "battles long ago," then wonders if it is a song of "today" about some "natural sorrow, loss or pain?"
The poet watches the woman as she reaps and sings, seeming as if she is at one with nature. To him, her song seems "unending." Finally, he moves onward but holds the memory of the song in his heart.
In summary, the poet tells us that the woman, who seems to him to be a part of nature, sings a sad but beautiful song. He doesn't know what the song is about but he does recognize its beauty, which stays in his soul long after the actual sounds have faded.
In this, Wordsworth shows himself to be a Romantic poet, idealizing both a common person and nature.