Dutch book

A dutch book is a set of bets that will guarantee you a profit no matter what the results are. It's like a bet where you are certain to make money.
Think of it like playing a game with your friends where you each have to put in money, and then the winner of the game gets to take all that money. If you could figure out a way so you would win no matter what, then you would always make money. That's what a dutch book does. It's a collection of bets that if you put them all together, you will always make money.