If something's at stake, it's at risk of being lost. When you play Monopoly, there's nothing at stake except your pile of colorful Monopoly money.
When nothing's at stake, you have nothing to lose — but when there's a lot at stake, you're taking a big risk. In a high-stakes poker game, there might be thousands of dollars at stake, and when you perform in a talent show you may believe your dignity and reputation are at stake. The source of this common phrase is unknown, although one guess ties it to an ancient (and cruel) practice of watching dogs fight a staked bear after placing bets.