Definition of hero fold
A hero fold is a fold made with a very strong hand. For the fold to be considered "hero" it must be a good fold. If you fold KK preflop and an opponent shows you AA preflop, it is a hero fold. If he had QQ, it's a bad fold. The hero fold is against an opponent you know well. If you have played several hands against an opponent and have a good idea of his range, the hero fold can be considered.
An example of a hero fold
You open 22 on the button and a very conservative opponent 3-bets you from the big blind. You call the bet. The flop comes 2-A-K and he bets on the flop. You hesitate between calling and raising, but finally you go with a call. The turn is another A and your opponent bets again. This time it's a very obvious call. The river gives a queen (Q) and your opponent pushes all in. Here you don't beat AA, KK, QQ, AK or AQ and your opponent is known to be conservative. You decide to fold your full hand into a 3bet pot for a hero fold. After your fold, your opponent shows you AK.