A dark bet is a bet announced for the next betting round even before the cards are dealt. To make a dark bet, the player doing so must be the first one to act in the next round.
Their announced bet is binding no matter what cards they get. In some poker rooms, a dark bet can also be a bet a player makes without looking at their hand.
For example, let’s say that one player raises and the other one calls. Then, before the dealer is ready to put up the cards on the flop, the player who raised during the pre-flop betting round announces that they want to make an extra $50 dark bet.
This means that regardless of which three cards come out on the flop, that round begins with a $50 bet.