It's not something that's available on all sites (Stars, for example, doesn't offer it) and it's usually only something you can get when you sign up for a new account. But here's basically how it works.
As you know, poker sites
charge you rake. They pay a percentage of that rake out to third party companies or individuals who encourage players to sign up with the site.
You'll see ads for these third parties all over the web, and normally they'll offer you some kind of incentive for opening an account at a site using their promo code: "sign up at XYZ site, enter the bonus code 'LOLAMENTS' and get a $50 bonus!" Deals like that are usually a one-time thing, but the third party continues to collect a percentage of the rake you generate on an ongoing basis.
Rakeback works in the same basic way, except that when the third party receives their percentage of your rake each week, they send a chunk of it back to you.
Say for example (and I'm making numbers up here) the site pays the rakeback company 30% of your rake per week. You play and play and play and pay $100 in rake for the week. The site sends $30 to the rakeback company, the rakeback company sends say $27 of that back to you, the player, and keeps the other $3 for themselves.
So long story short, it's a way for you to get back a chunk of the rake you pay to the site. If you have existing accounts that don't have rakeback you can always ask the site, but not many of them will activate it after you're signed up. Worth a try though.