the % chance that opponent will fold a better hand than yours to your bet.

ex. You have 67 and bet in LP preflop, BB calls. Flop is 389 rainbow. BB bets, and you raise. You technically have the worst hand if he has T9, with only a 32% chance of winning. But since you are representing a hand better than T9, the villain will fold to your raise. Fold equity is the percent chance that he will do so. In this case you probably need him to fold around 40% or so of the time for it to be a profitable semibluff.

Even if he doesn't fold though, its a good play because he will probably let you see a free river card.