The poet is unable to make out anything of the song the solitary reaper is singing alone in the field. It's in a language unknown to the poet. However, he finds her voice to be very mellifluous and believes the song has a “melancholy strain.”
The poet laments for there’s nobody to tell him what the words or lyrics to the song actually mean. To him, the notes of the song sound wistful and somber. So, he conjectures it could be about “old, unhappy, far-off things, / And battles long ago.”
Engrossed by the maiden’s song, the poet further imagines that it might also be about “some more humble lay, / Familiar matter of to-day?”
He further speculates that the song may be about “some natural sorrow, loss, or pain.” His restlessness seems to grow as he’s unable to find out the actual meaning or occasion of the solitary reaper’s song.
Will no one tell me what she sings?—
Perhaps the plaintive numbers flow
He ponders hard over the subject matter and theme of “the plaintive numbers.” Finding nobody to answer his doubts, the poet mounts “up the hill” bearing the music of the girl's in his heart. But about one thing he has no doubt that the song couldn't be about a happy or mirthful subject; it certainly describes some bitter experience or sorrowful subject.
"Whate'er the theme, the Maiden sang" the poet is never going to forget the experience of hearing such unmatchable melody.