Well every time you play a pot or a tournament the poker games take a small percentage of the prize pool (that's rake), that's how they make money. as the name suggests, if you get an account with rakeback then you can get some of what you paid back.
Stars doesn't have rakeback, they have a VIP rewards system which works in a similar way. if you look in your lobby you can win cash prizes if you get a certain amount of points. starts off with $10 bonuses and then goes to $50 and higher as you accumulate more points, which is kind of like rakeback. Should also note that you must have silver star to receive them.