The poem expresses the theme that simple moments can have extreme beauty and can soothe our souls for a long time afterwards if we stop and pay attention.
In this poem, the song of a woman reaping by herself in a field in the Scottish highlands catches the attention of the narrator, who is hiking nearby. He has to make a decision to keep moving or to pause and listen:
Stop here, or gently pass!
This seemingly simple choice is important to the narrator. He decides to stop and listen and doesn't regret this decision. Because he concentrates on hearing her, he is emotionally moved by the beauty of the reaper's sad, lamenting voice filling the air.
The narrator doesn't understand the words the reaper sings, so can only speculate what they mean, but this doesn't matter to him. The reaping woman's song conveys strong emotion and becomes, to the narrator, a sublime ("thrilling" and inspiring) part of nature.
Wordsworth (along with Coleridge) hoped to elevate everyday experience and the common laboring person in the minds of his readers, as well as to celebrate nature itself. This poem does both. Wordsworth, as he stated in the Preface to Lyrical Ballads, the book of poetry credited with launching the Romantic movement in England, also defined poetry as a "spontaneous overflow of powerful feelings" recollected in tranquility. At the end of this poem, he expresses a version of that idea:
The music in my heart I bore,
Long after it was heard no more.