Yes, you have the right idea. 'High-stakes' is an idom taken from the world of gambling: your stake is your "apuesta". The higher the stakes are, the more you risk losing. If an exam is seen as 'high stakes', it is taken seriously. People put a lot of time, effort and money into trying to pass it, and risk losing a lot if they don't pass. However, if they do pass, they have a lot to gain.