Rake is a fee taken by poker rooms from each pot as their profit. It typically ranges from $0.05 to $3 per pot, depending on the limits you play and the size of the pot.
Poker rooms track exactly how much rake you pay in each pot, but this amount is not visible in the poker client. You only see the total rake taken out of a pot.
Rakeback is a loyalty program for online poker players. With rakeback, you can receive a refund of the rake fees that poker rooms have already taken from you.
To get rakeback, you need to sign up with a rakeback site, choose a poker room offering rakeback, and follow the sign-up instructions on the poker room's rakeback page.