The title of Feathers refers to an Emily Dickinson poem that serves as the book's epigraph (see the "Epigraph" section). The whole idea of the poem is that hope is a "thing with feathers" and that it persists in the face of all sorts of adversity… which is what Frannie comes to realize over the course of the novel. This is her big revelation as she grows up in the book.
The book is about how Frannie finds hope in the little things in life, like in the love that she has for her family, or in her friendships, or in the abatement of racial tensions that she sees in her own school. All the progress may be small, but it's still there—and that's a beautiful, light, hope-filled thing. Feathers are connected to flying, and in Feathers, Frannie learns to worry less and let her hope soar.