Fixed-odds betting is when you bet on something happening and the amount you will win is already decided, or fixed, before the event happens. Think of it like a game of marbles - you make a bet with your friend that you will win the game and they agree to pay you a certain amount of marbles if you do.
For example, let's say there is a football match between Team A and Team B. You think Team A is going to win, so you place a fixed-odds bet of $5 on Team A winning. The bookmaker, or the person you are betting with, has already decided that if Team A wins, they will pay you $10 (including your original $5 bet). If Team A loses or ties, you lose your $5 bet and don't get any money back.
The odds, or the likelihood of a certain event happening, are already calculated before you place your bet. This means that you know exactly how much money you can win before the event happens.
Overall, fixed-odds betting is a way to make a bet on something happening and know exactly how much you stand to win or lose.