Its a scale from 1-100 where 50 is (approximately) the average score based on the average stats of the 252 40+ minute super rugby games played by 1st 5's in 2014. So if a player gets the average stats, he gets a score of 50, gets more than average its above 50, if he gets less its below. (Side note, in the early running of 2015, average stats across the board are down from 2014)
There are 11 categories a player can contribute to his overall rating, tries, running, line breaks, turnovers, tackling, goal kicking (not included obviously for players like Matt Toomua), territory kicking, possession kicking, kick errors, attacking work rate and defensive work rate. Each category is broken down into different stats has a different weighting to match its overall impact on the game, e.g. tries/try assists are worth more than line breaks/assists. Each category has a minimum and maximum amount of points, so a player who only does one thing, but does that one thing brilliantly he won't get a brilliant score. If someone scores 6 tries in 7 games, but misses 40% of his tackles and 50% of his kicks it's going to balance back down to average.
The stats it takes into account are minutes played, tries, try assists, total touches, number of runs, metres run, line breaks, line break assists, defenders beaten, turnovers, tackles made, tackles missed, kicks made, kicks missed, (territory kick metres, number of territory kicks, possession kick metres, possession kicks, number of penalty kicks to touch, and kick errors).
The bracketed stats require difficult and unique stat collection and so aren't included for super rugby, so you'll notice that an excellent out of hand kicking game, like the one Sias Ebersohn had in the forces win over the Waratahs in round one won't rate as highly as they should. That's purely financial decision and if it were up to me those stats would be included. If the public get out and support and it becomes financially viable, then it will get added back in later on.