There are lots of really complicated mathematical forumlas to explain it...but basically i dont understand it and its really complicated so ill give you a rough explanation:
Player X has 1000 points
Player Y has 1000 points
Player X wins vs Player Y...Ratings will now roughly look something like this
Player X has 1010 points
Player Y has 990 points
now imagine Player X (now on 1050) plays a noob who has a rating of 100 (player Z).
Player X plays agaisnt Player Z and wins, ratings will know look like this:
Player X has 1051 points
Player Z has 99 points.
IF Player Z had won though, ratings would look like this:
Player X 1000 points
Player Z 150 points
K that wasnt so simple. Basically, Players with high ratings that beat players with low ratings will get very few points. However, players with lower ratings that beat players with high ratings will get lots of points (reward for beating a better player) and the higher player will also lose lots of points.