An elegy is a poem that expresses grief or sorrow that someone has died. In "She Dwelt Among the Untrodden Ways," the speaker laments a "maid" or young woman named Lucy who has died. Therefore, the poem is an elegy.
In simple language, the speaker tells us that Lucy was an individual who knew very few people and had nobody to sing her praises. He compares her to a violet, blooming by a "mossy stone," so that it is hard to see. He says she lived "half hidden."
In the final stanza, the speaker emphasizes once again how obscure Lucy was. We are told "she lived unknown." Nevertheless, the speaker knew her and expresses sorrow over her passing away, saying the "difference to me!"
This simple poem alludes to Thomas Gray's "Elegy Written in a Country Churchyard," a longer set of verses that commemorates all the unknown and insignificant people buried in the humble graveyard of a country church. Gray famously states that:
Full many a flow'r is born to blush unseen.
This idea is picked up by Wordsworth, whose speaker's Lucy is like a violet "half hidden from the eye."
Like Gray, Wordsworth asserts that even a humble person has great worth. Wordsworth's project as a Romantic poet was to shine a light on the simple, ordinary people of the world who were so often either overlooked or ridiculed by poets and show them in the best possible light. In this poem, he generates sympathy for Lucy by depicting her as a quiet person who meant a great deal to his speaker.