The fee the house or site takes from the pot.
It is set like "5% of the pot, capped at 3bb." The higher the rake, the less a small pot is worth playing, so tighten your range one step.
At the table
At a table with a rake of "5% of the pot, capped at 6," a pot of 100 has 5 taken by the house, and the winner receives 95.