If I call someone's push, I have one way of winning the hand. This is by showing down the best hand.

If I push into someone, I have two ways of winning the hand. I can show down the best hand, but there is also the possibility that he will fold. This is fold equity.

However, if I push 450 chips from the button and the BB is 400, then I don't have fold equity. A reasonable BB will call me with any two cards, so I only have one way to win the pot again.

Basically remember that it takes a far stronger hand to CALL a push with than to push it yourself (the "gap concept"). And don't let your stack dwindle so far that you no longer have fold equity. You should be pushing all-in long before that.