Probably Wordsworth had the idea of writing this poem when he went to Scotland in 1803. There he saw a solitary girl that reaped and sang a song in a strange language (maybe Gaelic) that he couldn’t understand.
While he listened, he imagined strange and exotic lands.
In this poems Wordsworth’s speaking to a wayfarer that he imagines to have met.
He sees a girl in a field, alone. This is what every man is, according to the poet: alone. She’s singing a sad song, probably because she is thinking about someone she has lost: a lover for example. While she’s singing, she’s reaping the strain. The sound of her voice is all around the vale.
Neither a nightingale could sing better than her, whose song would be pleasant to the wayfarers of Arabia. In fact her voice makes dream everyone who’s listening to it.
The poet looks at her, quiet and still, and he’ll always keep the memory of her beautiful song in his heart.