If you play poker for real money, the poker rooms will levy a fee, which is calculated by a certain percentage (in most cases between 3% and 5%, and with a maximum of 3-5 $) of your pot. This fee is called rake.
For a layman the mentioned amounts seem to be minimal, but for a player who is playing several times a week, this fee can rise to a multi-digit sum which has to be delivered to the poker websites.
Through rakeback you get, differing from offer to offer, a certain percentage of the paid fees (rake) back. If you register at ddonk.com you get a rakeback, regardless of the limit you play.