Hey, I'm Nicky Numbers, I'm a professional poker player and I'm going to explain fold equity. Fold equity is the value gained by taking an aggressive action that sometimes results in your opponent laying their hand down and you winning the pot uncontested. Whenever you're playing a hand you have a certain amount of equity and your opponents have a certain amount of equity.
Equity is just a fancy way of saying, 'your chances of winning.' So, if I'm in a hand and I have an 80% chance of winning and you have a 20% chance of winning, then you have 20% equity in the pot. If I take an aggressive action by betting, or raising, or re-raising, that makes you fold your hand, you give up that 20% that you were rightfully entitled to had all five cards come out and we had gone to showdown.
In other words, if we were all in at this point in the hand you would realize the true equity of your hand and I would realize the true equity of mine. I'm 80%, you're 20%, so I would win four out of five, and you would win one out of five. But when I take an aggressive action that makes you fold you give up that 20% and I gain it. That's fold equity.