The rating system monitors a player’s perfomance by his/her winning probability per tournament.
It calculates your winning probability by finding out the average rating of the opponents from whom you have won and based on how high or low your rating is from the average it calculates your winning probability for that tournament. If you perfrom better than your calculated probability then your rating will increase and if you perform below it your rating will decrease.
Your rating remains provisional till you have played 50 games. This means that your increase or decrease is not accurate till the required number of games are played
If you stay inactive for more than 2 months or have not completed 50 games then your rating will not be ranked on the website but will remain in the RATINGS file, which gets updated after every rated event, for future reference.