Fold equity
The value of the chance your bet makes a better hand fold. What turns a draw into a semi-bluff.
You're on the button with 9h-8h. Flop comes Kh-7h-2c — no pair, nothing made, but four hearts are showing, which leaves nine in the deck and a flush by the river. Big blind checks. You bet half pot. Now the hand wins two ways. Big blind holds Q-J or a weak pair, reads the bet as too strong, and folds — pot's yours, no turn card needed. That extra win, the one your cards had nothing to do with, is fold equity.
A bare draw, bet on its own terms, has to clear pot odds or you're burning chips. Folds change the math. Every time the bet takes it down, you win the whole pot no matter what the flush would have done, which is why a semi-bluff can print even when the called-down math looks thin. Stack depth and position push it around too. A short stack shoving into a big pot hands the opponent a cheap call, so folds dry up no matter how scary the bet looks.
The beginner error is assuming the fold is there just because the hand has outs. Against a calling station, or in a multiway pot where three players have to fold instead of one, the pressure's gone. Bet a draw into a field that won't lay it down, or into someone already pot-committed, and you've turned a fair hand into a naked bluff with no backup.