The definition of fold equity is the value you get from the times your opponent folds. So if there is $100 in the pot and you semi bluff push all in (for $100) and your opponet folds 50% of the time then your fold equity is $50 (0.5xpot). If he wins 75% of the time he calls you then your equity for the times he calls you is $200x0.25 + -$100x0.75=-$25. So your total equity is your fold equity added to the equity you have when you get called = $50 + -$25 = $25 making the move +EV overall.

Summary

Fold Equity = (% of the time your opponent folds) * (the amount of money you win when he folds ie pot size)