William Wordsworth's poem "The Solitary Reaper" describes a maiden reaping in the fields, meaning harvesting the fields using a sickle, and singing as she goes about her work. A central theme in the poem concerns the overwhelming beauty of something that seems as mundane as either harvesting or singing. In other words, according to the poet, the mundane is not truly mundane but overpoweringly beautiful. Hence, in the final stanza, the poet says, "I listened, motionless and still," in order to describe the effect the maiden's singing had on him. He was so overcome by her singing that he was unable to move; he especially refused to move so that he could hear her without missing a note.
The beauty of the maiden's singing is especially expressed in images of birds famous for their singing. For example, in the second stanza, Wordsworth compares the maiden's singing to that of a nightingale's, saying, "No Nightingale did ever chaunt [chant] / More welcome notes." He further compares her singing to that of a cuckoo bird's, saying that her voice was more "thrilling" than even that of a cuckoo's voice in the spring.
Beyond the beauty of her voice, what else makes her singing so captivating is the words she is singing. Apparently, the speaker in the poem is unable to understand the words she is singing for he asks, "Will no one tell me what she sings?," possibly because she is singing in a foreign language. Yet, despite not being able to understand the words, the speaker is still able to hear a great deal of sorrow in her voice, leading him to speculate on the subject matter of the song, which adds to the thrill. For example, he speculates she is singing about unhappy times past, such as battles from the past, or "[s]ome natural sorrow, loss, or pain" that she has already suffered and may suffer again.
Hence, according to the speaker, the beauty of her voice coupled with the sorrow in her voice and the mysterious subject matter of the song makes her singing extremely captivating. The speaker is so captivated by something as mundane as her singing as she goes about her mundane work that he stands frozen in place in order to continue listening to her.