short version is that FE=the % of hands in his range he folds.

In a hypothetical situation for shortness and ease, If a 20/12 opponent Raises UTG and you give him a range of AQ+, 22+ and suited Ax and some connectors 89s+ (This is way wide if he's at least a little positionally aware) That's 170 hands

He then Cbets a TsJs3h flop his range is still wide and he could have just about anything depending on his Cbetting frequency. For arguments sake we say he always Cbets his UTG hands, but folds anything that missed the flop when raised. He's only continuing with Axs spades SC's with spades 33, TT+, and most of the SC's, and folding all the rest. That's about 98. thats 58%, so we can estimate our FE at about 42%

If we shove he folds 42% of the time. This means we need to factor things like, How much equity we have in the pot, what we win if he folds and how much we win when called.