It's the average distance of the players of a team to their main buildable. It starts at zero because there are initially no players. The idea was to determine camping and rushes, but I should replace it with something better suited, such as some form of ratio between distance to your own and the enemy base.
In the current plot, I use the 1000 qu line to distinguish between inside base and outside base, even though this is an arbitrary distance. The niveus match shows what happens if humans don't leave their base:
The humans average distance to the RC is below 1000 qu during most of the match. They are constantly attacked by aliens, who have control over the whole map and can push their mining rate to a very high level by expansion. The alien builder uses all available build points for new forwards and defenses, which is one of the reason the aliens confidence grows steadily. The other reason, which kicks in around minute three, is the fact that they get confidence for killing non-naked humans, too. The humans don't profit from either because they can't expand (and decide not to build inside their base either) and don't get any confidence for killing aliens, since the fight is going on inside the friendly base. Just like in Tremulous, humans get a lot of credits for defending the base but they are not rewarded with a stageup. The aliens are able to take out a few structures around 3:30 which leads to an early stageup at minute four. This is devasting for the humans: Recovery doesn't work for them because they had never enough map control to mine resources, so they are out of build points at minute six. The reactor goes down fourty seconds later (average distance to base is zero).