The "author's wheel," introduced in the Prologue, is an analogy of a Ferris wheel. Up close, this wheel represents a circle of time (specifically a year). But the bigger idea presented in this analogy is that the wheel represents the circle of life itself.
In the context of setting, the center, or hub, of the wheel is the "the wood," a dense forest near the village of Treegap which has no road leading through it. This wood is described as having an appearance of an "otherworld" and is owned by the Fosters, who live at its edge. At the end of the Prologue, however, the author presents a caution to the reader that gives insight into the deeper meaning of the center of the wheel. She suggests that the hubs of wheels are fixed points and should be left undisturbed. Thematically, this idea foreshadows the main point of the entire story.