Outs are the cards still in the deck that, if dealt, would improve a player’s hand enough to win. For example, a flush draw has 9 outs (the remaining cards of that suit). Each out represents approximately 2% probability of completing the hand on one card. The rule of 2 and 4 is a popular shortcut: multiply outs by 4 on the flop or by 2 on the turn to estimate equity.