[url=https://en.wikipedia.org/wiki/Centrality#Betweenness_centrality]This should help explain[/url]. [quote]Betweenness is a centrality measure of a vertex within a graph (there is also edge betweenness, which is not discussed here). [b]Betweenness centrality quantifies the number of times a node acts as a bridge along the shortest path between two other nodes[/b]. It was introduced as a measure for quantifying the control of a human on the communication between other humans in a social network by Linton Freeman[17] In his conception, vertices that have a high probability to occur on a randomly chosen shortest path between two randomly chosen vertices have a high betweenness.[/quote]Nodes = users. In the picture the more blue the circle is, the greater its centrality to the network. Another way of looking at it is how well a given node connects to other nodes. For example, if you looked at a red node in the bottom right hand corner and counted the number of other nodes you would need to travel through to get to the opposite side, it's probably shorter to go through the centre where the blue nodes can more easily connect you to the other nodes than it would be to travel through all the red nodes around the outside.