The rake is the portion of each pot that the house keeps for itself as a fee for providing the games. The rake is typically expressed in the form “X% up to Y.” X is the percentage of the pot that the house retains while Y is the maximum amount taken.
For instance, if the rake is 5% up to $3, then this means that the poker site extracts 5% of the pot for itself, but the maximum rake per hand is $3 even if $3 is less than 5% of the final pot size. This upper limit on the amount of rake is called the “cap.”
It's usual for the rake structures to vary based on stake level and the variant of poker being played even at the same site. In tournaments and SNGs, the poker operator takes a tourney fee rather than a rake.
If a particular site has low rake, this means that it is removing very little from each pot, meaning that there's more of the money remaining in the middle for the winner of the hand to claim. Low rake leads to better financial results at the tables for both winning and losing players.