The stakes are what you can win or lose in a competition or a game. They can also be the things that are important in a situation.
When the stakes are high, it means that what you can win or lose is very important.
Examples:
The stakes are high in this game.
What are the stakes in this bet?
She knows the stakes are high, so she tries very hard.
The stakes are so high that I'm a little scared.
The stakes in this race are higher than in any other race.
He knows the stakes, so he is careful.