Ok I'll take a shot. Fold equity is a matter of estimating. Let's take a look at a hand I played recently.
I have
![The Ten of Spades [Ts]](https://www.pofex.com/images/smilies/Ts.gif)
in LP and call a small raise. The flop comes down
![The Nine of Diamonds [9d]](https://www.pofex.com/images/smilies/9d.gif)
. Ok, I flopped the straight AND flush draws.
Now. The original raiser bets the pot which at this point is $5, I push all-in. Here's the idea. Against a top pair hand like AQ, I'm actually a favorite in this hand, and against a set I'm even money. So obviously, I don't know what he has, but I can make some inferences, and thus estimate my fold equity. So lets say with whatever range of hands he could call me with I'm even money. In that case I win and lose an even amount of money in the long run, thus my EV is exactly 0. However, if he folds even 20% of the time ( a conservative estimate), then 1 out of 5 times I win the $10 which is currently in the put, thus giving me a +EV situation. This is the basic idea of fold equity. Which in turn is the basic reason for agression in NL, if you can get your opponents to fold their hands a given amount of the time, then you win the money which is already in the pot which can turn an -EV hand into a +EV situation, this is basically the entire premise of semi-bluffing as well.
Hope that helped.
-Justin