Rake is a commission fee taken by the one operating a game. Later, the money covers the whole operation of running an online casino.
There always is a small amount taken from the winning pot, deducted out of every hand in the game. Usually, it's somewhere between 2.5% and 10%, so you should check it before you start playing with your chosen app. The maximum amount is most often predetermined, so it's not going up endlessly.
Some providers follow the rule called 'no flop, no drop,' which means that they don't take percentage rake when a hand doesn't have a flop.