Wordsworth's speaker makes this statement in the first stanza, speaking to himself. He says he has seen a young woman "reaping and singing by herself." He then says to himself, after the hard stop of a semicolon, "Stop here, or gently pass!"
He says this to himself because he has to make a decision. He can stop and listen, or he can quietly pass by the reaper, missing her singing. As he notes, he stops, saying, "O listen!"
He is well rewarded for taking out the time to stop and listen to the song the young woman sings as she reaps. It is so beautiful the speaker feels transported. To him, the singer becomes a creature of nature, like a bird but with a song even more beautiful than that of the nightingale or the cuckoo. The speaker is well rewarded for stopping and listening. Although he cannot hear the words of her song, the haunting, "plaintive" melody moves his soul. The song is also not only a momentary pleasure for him but a memory he will carry with him. He says,
The music in my heart I bore,
Long after it was heard no more.
The poem is a quintessential Wordsworth piece. It exalts a common laborer, a woman reaping a crop. This is the kind of person largely overlooked in poetry before Wordsworth and Romanticism appeared on the scene. He elevates her into something lovely and natural by comparing her song to that of birds. He also shows how a simple event can live on in memory, providing repeated solace and delight.