What is Rakeback?
To understand what rakeback is, you must first understand what rake is. Rake is the way that online poker sites make their money. Everytime players are involved in a cash game where a pot reaches a certain amount, the poker site takes a small percentage of the pot for providing players with their services. This is the same for tournaments as when you sign up to play a tournament you will often see $10+1, which means there is $1 in rake going to the site.
Rakeback means that you get a percentage of this money you pay to the site paid back into your player account. This was originally used to attract high volume players to poker sites, but has since become available to any player at various poker sites, including PKR.com.