Fold equity is the amount of the pot you expect to win when the other players fold.

In the early stages of the tournament the blinds are low so you play good hands and hope to stack someone.
In the later stages of the tournament the blinds get huge so you cant afford to wait for a monster hand. Blind stealing can keep you afloat.

e.g. You are delt QJ in the CO.
The blinds are medium stacks but smaller stacks than you. You notice that they seem to be playing very tight at the moment, as though they are scared of not finishing in the money. There are 2 shortstacks at the table (acting before you) who are just waiting for a hand to go allin with.
It is folded to you, and you decide you have a good chance to steal the large blinds (your folding equity is high) so you raise with QJ even though you might just limp/fold when the blinds are small.