This poem is yet another example of a Wordsworth poem that is based around one of his walks in nature, but this time in Scotland, rather than in the ubiquitous Lake District that was the focus of so much of Wordsworth's poetry.
Whilst walking, Wordsworth comes across a "solitary Highland Lass" working by herself in the field, reaping and singing. The "melancholy strain" of her song and the way that it seems to make the vale "overflow" with its sound causes the speaker to stop and continue his perusal of this solitary reaper without her being aware of his presence.
The second stanza compares the sound of the reaper's song to a nightingale's song and then to the cuckoo, finding both wanting in comparison to the beauty of the reaper's song. The third stanza explores how the song's meaning cannot be deduced, and the speaker speculates that perhaps it may be of "old, unhappy, far-off things," or perhaps it captures some "natural sorrow, loss, or pain" that is eternal.
Either way, the last stanza explores the way in which the song is sung as if it does not have any ending, and how the speaker, as he leaves the sight, hears the song in his heart long after he left the solitary reaper.