Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The epigraph is a line from a poem that appears in The Lord of the Rings: Fellowship of the Ring, a book by J.R.R. Tolkien, and for our purposes, it foreshadows a major theme in this book, as each of the main characters wanders through her summer, eventually finding a better understanding of herself.
The epigraph implies change—wandering is nothing if not movement—and change certainly awaits each member of the Sisterhood. But despite their meandering journeys away from each other, the girls eventually all make their way back home. They aren't lost, and by the story's end, not only have they found themselves as individuals, but they also come back together. Until next summer, anyway.