You're in for a big one - Tradução em português

Olá pessoal,

No texto abaixo eu não consegui entender perfeitamente o significado da última expressão: You're in for a big one.
Eu fiquei na dúvida do que isso poderia significar, se seria um problema ou se a pessoa estaria indo pegar peixe grande..

There's another fishing pole for you. You can try to catch a fish yourself. Once the fish is on the hook, you can start
to reel in the line very slowly. No, no, not that fast! You have to do it a lot more slowly. Otherwise the fish starts to
fight. And then they sometimes get off the hook. Yes, that's much better. You're in for a big one.