You can calculate fold equity as the % of the time you think a player will fold to your bet, raise or re raise. If you see a player who calls bets pre OOP (out of position) then checks when he misses the flop, your FE is very high if checked to OTF. So you could bet into this player with any two cards and expect him to fold a large % of the time. You can also use FE combined with your equity in the hand. Like, if you put a player on a strong high pair, and you have a straight and flush draw, you can raise or re raise (of course, other factors must be considered but for simplicity's sake) and if you are called, you still have outs, so if you think he is capable of laying down a strong hand some of the time to you, you have extra equity in that he may just fold. There are many factors that influence your FE: Your image, the texture of the flop, action so far, relative position of all players involved, the number of players involved are a few, and of course you only have FE vs players who know how to fold.
EDIT: lol, I started typing an answer and then someone came over and I was distracted, finished and voila..... MANY other answers in a few minutes...