In "The Solitary Reaper," the speaker uses apostrophe which means that he is addressing some absent person, idea, or thing. In this poem, this could be the speaker himself, nature, or even the reader that he is addressing. He describes the sight of a Scottish girl reaping the grain while singing a melancholy song. He compares her song to the sounds of the nightingale singing in a far away Arabian desert and to the cuckoo-bird singing "Among the farthest Hebrides." In this comparison, the speaker suggests the girl's song transcends space and time, he is so overcome by its natural beauty. The fact that the girl is harvesting grain makes her connected to nature and therefore, her song, in his imagination, transcends the boundary between the girl and nature. In these lines, nature, "the Vale," is made profound by her song. Nature is personified; it is revitalized by her song and the entire experience for the speaker becomes profound.
O listen! for the Vale profound
Is overflowing with the sound. (7-8)
Because he cannot understand her dialect, he is free to imagine that the content and quality of the words she sings also transcends normal description. The maiden never notices the speaker's presence. As he leaves, the song leaves with him in memory, literally showing how the song transcended (moved beyond) the time and place where/when it existed (was sung).
Wordsworth often wrote about contemplating everyday events and literally thinking or imagining his way to understanding these things with more significance. The girl's song was a physical event which, as he carried with him in his heart, could move beyond it's own physical manifestation. Her song is also a metaphor for Wordsworth's own imagination. That is, he could start with this one thought and progress to other ideas, literally transcending the thought he began with. This is a practice we do without thinking about it; we have one thought and, usually without realizing the progression, we are thinking about something entirely different only moments later. Wordsworth believed the imagination was a kind of creative contemplation whereby one could think about one thing (often dealing with nature or humanity), even a mundane, everyday thing, and come to a renewed sense of truths about existence or the creativity of human thought.