Fold equity
Fold equity is the extra value a bet or raise earns from the chance your opponent folds, letting you win the pot without having the best hand.
Every time you bet or raise, there are two ways you can come out ahead: your opponent calls and you win at showdown, or your opponent folds and you scoop the pot right there. Fold equity is the value of that second path, the share of the time your aggression simply makes them give up. It is what separates betting from just checking and calling, because checking can never make anyone fold.
It comes up most when you are bluffing or semi-bluffing. A pure bluff relies almost entirely on this, since if nobody ever folded, a hand with no showdown value would be worthless. A semi-bluff, like betting a draw, stacks two chances to win: opponents folding now, plus your draw still hitting when they call. The more likely a player is to fold, and the bigger the pot you would win, the more this is worth.
As a beginner, the practical takeaway is to ask who you are betting against before you fire. A tight, cautious player gives you lots of folds, so bluffs work well; a loose player who calls everything gives you almost none, so save your chips and bet your strong hands for value instead. You also tend to have more of it with fewer opponents and when you act after them rather than before.
Example
You hold 9♠8♠ on a K♠6♠2♦ flop, giving you a flush draw but no made hand yet. Betting here is a semi-bluff: a cautious opponent often folds to your bet (that is your fold equity), and even when they call, you can still win the pot when a spade completes your flush on the turn or river.