You start your first game in a premier event with a rating of 1600.00.
If you win, you gain points.
If you lose, you lose points.
If you beat someone with more points than you, you get more. If you lose to someone with less points than you, you lose more.
So your rating is a scale of how many games you have won, and against how hard of opponents you have played against.
Each event also has a "K" value that determines how much your scores go up by. I have played in two Battle Roads, whose K-values were 4. So I only have 1606 despite having a decent record. However, most people have played in several CCs, have much higher ratings because CCs have a K value of 16.
Get it?