Rakeback is used by some poker affiliates. It works like this:
When a player plays poker at an online poker room, the poker room charges a rake for letting players play there (that’s how they make money).
So, what some affiliates do is that they can track how much each player plays at an online poker room and how much rake that player paid. They then simply pay this player some % of their rake back. Players like it because they get some money back and some affiliates do it because when offering rakebacks, it’s easier for them to signup tons of players and eventhough they then only make a smaller % as income of the rake, the number of players accepting these deals makes up for it.
Hope that helps,
Thomas