Taking a rake in poker refers to the percentage of the pot that the house takes in compensation for hosting the game. Why did Molly take a rake in Molly's Game? According to the film Molly's Game, Molly Bloom started raking her games to pay players out.
Poker is a player-versus-player game where players wager against each other rather than against the house. Casinos thus require a means to generate revenue. Rake is how they generate this revenue and offset the costs of facilitating games.