I feel like a moron for having no idea what any of this means.
When you win a large pot say, $60, the site is going to take a rake of $3 from that (5% up to $3 usually). That's how they make their money. With Full Tilt, you can get rakeback of 27%, so on a weekly basis 27% of the rake paid will be deposited back into your account. It's a nice bonus and for many players is the only way they break even / profit.
So basically, the affiliate that sent FT the customer gets ~35+% of the rake paid for life, rakeback affiliates are nice enough to kickback 27% to you. I use "rake the rake" but there are many.
If you get linked to a non-rakeback affiliate, you will never get rakeback, and they'll close any new accounts you try to create.