What is the definition of "hub" as it is used in Tuck Everlasting?

As it's used in the story, and elsewhere, a "hub" is literally the center of a wheel (the solid part to which all the spokes are attached--here's a drawing) or, figuratively, a "hub" is the center of the action. Other events and activities revolve around the "hub," just like a wheel revolves around its center. For example, you could say that the cafeteria is the hub of social activity in the mornings before school starts. Or, you could say that your college plans are the hub around which you organize your academic, social, and family activities.

Let's check out this word's use in the prologue of this novel:

"But things can come together in strange ways. The wood was at the center, the hub of the wheel. All wheels must have a hub. A Ferris wheel has one, as the sun is the hub of the wheeling calendar. Fixed points they are, and best left undisturbed, for without them, nothing holds together. But sometimes people find this out too late."

As you can see, the narrator builds on the meaning of "hub," pointing out how the hub stays in the same spot while other things are in motion all around it. The narrator is saying that, like the sun, or like the center of a Ferris wheel, the wood in this story is the very center and it remains still, and all the action of the story will revolve around that wood.