In order to understand what this statement means, one needs to understand how data networks work. Data networks are made up of links.
Links are simply physical or logical connections between nodes.
Think of each link as one block of a city street, and nodes as intersections of those streets. Together, the links and nodes make up a network. Ethernet and Frame Relay are two examples of traffic control systems that control the flow of data over any given link. In addition to a traffic control system for each link, there is an overall traffic control system for the overall network. The network protocol basically controls the flow of traffic among links, i.e., how and when traffic leaving one link will flow onto another in order to get that traffic to its ultimate destination.