A hub is the center of a wheel or the center of some kind of activity. If all of an airline's flights go through Atlanta, you'd say the southern city is their hub.
You know how a lot of activity is called a hubbub? That makes sense when you consider that a hub, in a wheel or otherwise, is the center of whatever's going on. Southern California has long been a hub of the computer industry, and Detroit used to be the hub of the auto industry. Busy airports like the ones in Chicago and Atlanta are called hubs. Any hub is important, because it's right in the middle of all the action.