Active Rating ranks you how active you are since Sunday, September 14th 2008.
It takes the average ingame time of players with greater then 15 minutes ingame. It does not count time from gm chars, LuLz chars, shout bots, AIO buffers and other off-the-chain stuff like that.
If your time is greater then half the average ingame time you get +1 point.
If your time is greater then the average ingame time you get +2 point.
If your time is greater then 2x the average ingame time you get +3 point.
Then it takes adds your ingame time / average time to your total points.
If your level is less then 80, you'll only get half of the total points.
If your ingame is greater then 3 minutes but less then 15 minutes you get 0 points.
If your ingame is less then 3 minutes you get -1 points.
If your ingame time is less then 1 second you get -2 points.
If you are inactive for more then 2 weeks you get -5 points.
Your rating will not go lower then 0.
It is useful to see who plays a lot and who does not. Also if your a clan leader it can help you pick and manage active members.