There are sites which offer rakeback. However, since you generally have to sign up to a site via an affiliate that runs the rakeback deal, the rakeback sites compete with CardsChats links, so I'm not posting any links to any such sites.
Rake is what is taken from each pot by the cardroom. This is generally something like 5% of the pot, and normally capped at something like $3 (for online games, although this varies from site to site). Rakeback is a deal where some of this earned rake is returned to you.
In World Poker Exchange's 100% rakeback deal, you simply add up the rake taken while you've been playing, divide it by the number of players at the table, and that's your share of the rake that will be returned. With other rakeback deals, where you get, say, 30%, you reduce your share of the rake to that percentage - so if the rake was $1, with 10 players, and 30% rakeback, then your share of the rake is $0.10, and you get 30% of that returned to you - so you get 3c.
As you can see generally it's not a huge amount in a full table, but it can add a couple of dollars an hour to your earnings at each table you play, which can make the difference between a profit and a loss, and can add up more noticeably if you multi-table.
One thing to note, however, is that most of the deals deduct the bonus from any rake you've earned before getting rakeback, so if you're clearing a bonus on a site, generally you're not earning rakeback as well... and since the rate at which you clear a bonus is usually more profitable than the rate at which you earn rakeback, initially it's usually better to make sure that you have a bonus to clear, rather than worrying so much about rakeback deal. Also, some of the most generous deals are at sites which don't have that much traffic, and soft competition and game choice is more important than a rakeback deal, too.