"Hub and spoke" is a term in air transportation to describe a common method for an airline to organize their flights. Airlines will have hubs in a few cities where most of their flights will originate, with spokes out to non-hub cities. Much like the hub and spokes of a bicycle tire. When flying from one city to another, you will typically have a connecting flight through one of the airline's hubs.
This system can also be implemented with other forms of mass transit, and cargo transportation.