An allegory is a really long—say, book-length—metaphor, where various concrete things in the text stand for various immaterial concepts. (Need a good example? Check out George Orwell's Animal Farm, in which animals stand for political viewpoints.) It's a bit musty as a literary convention, but Woolf manages to use it in a subtle way. Let's check it out:
What if the campuses of Oxbridge and Fernham stand in for men's and women's educational opportunities? What if the British Library represents all of the angry anti-woman rhetoric out there? What if the narrator's home library is a trip through a few thousand years of history and literature? And what if her window looks out toward the writers of the future who will be able to forget about their own sex as they write?
Yeah, it's looking a lot like an allegory to us.
Using allegory lets Woolf illustrate of one of the themes of the text: documenting the little insignificant details of everyday life is the way to get at the largest, most important things. "What is meant by 'reality'?" she asks. And then goes ahead and answers her own question: "It would seem to be something very erratic, very undependable—now to be found in a dusty road, now in a scrap of newspaper in the street, now in a daffodil in the sun" (6.16).