If it doesn’t, the bookie keeps their money. If the bet wins, the customer gets paid out. If they do, they then just need to decide on their stake. The layer/bookmaker sets odds on a player/team/horse etc to win a game/tournament/race and the backer/customer chooses whether they want to place at those odds. A backer thinks Novak Djokovic is going to win the upcoming US Open and wants to bet on him a layer is happy to offer odds on that happening and taking the view, so to speak, that he won’t win it. The person laying the bet is saying that it won’t happen.

The person backing the bet is saying that an event will happen.