Betting is simply predicting the outcome of a game or event and finding someone else prepared to take the opposite view. Each side puts some money down known as their stake. You lose your stake if you guess wrong but if you guess right you get it back along with the amount the other person put down.
Bookmakers provide odds on all the outcomes of an event and will pay out at those odds if you select the right one. Betting on fixed odds like this means you can only ever lose what you initially stake.