"Don't judge a man until you've walked two moons in his moccasins."
Have you ever heard of that phrase, "try to put yourself in someone else's shoes?" It basically means, "try to imagine what it must be like to be someone else." Why would you want to imagine what it must be like to be someone else? Well, so that you can learn not to judge them so quickly, so that you can learn to empathize with them. How do you empathize with someone? You use your imagination to figure out what it must be like to be them and to experience what they are experiencing. Well, that's pretty much what Walk Two Moons is all about.
The epigraph, "Don't judge a man until you've walked two moons in his moccasins," also appears in the actual story, scribbled on a piece of paper and left at Phoebe's door. Phoebe and Sal think that a lunatic who plans on killing Phoebe and her family is responsible. This saying becomes the heart of this whole story. In fact, the book's title even comes from this saying.