The term backer in regards to betting is simply the person who places the bet, “backing” their team to win, an outcome to happen, or any other event which is bet on. This is the backbone of betting, with any exchange needing a backer and a layer – someone to bet, and someone to cover the bet.
There are examples of backers in all betting. When someone thinks a team will win a game, they back the bet, becoming the backer. The bookmaker offering this bet is the layer. If the team loses, then the layer keeps the stake, but if the team wins, then the layer pays the backer based on the agreed odds.