Like other "Lucy" poems, this one shows Wordworth's belief (based partly on the writings of Rousseau) that Nature is the best teacher, and anyone reared entirely by Nature would be without evil. So Lucy is this kind of ideal, uncorrupted person (also, like a lot of men of his time, Wordsworth wasn't too crazy about higher education for girls!) If there was a real Lucy she has never been found - some people think the poem may be a partial expression of his love for his sister Dorothy.
You might get some more ideas from this essay.
You might get some more ideas from this essay.