This is the money used to wager on the success or failure of a particular event, or in other words, the money used to place bets. It represents the total investment in a given wager for a sporting event.
A bettor's stake in the game is determined by how much money they bet. The term is also commonly used in modern American English to represent ones overall investment (time, money, sweat, etc.) in a given project or enterprise. The term comes from the history books, when land was the item most coveted in the US. Plots of land were marked with wooden stakes in the ground to denote where one plot ended and the other began. The term stuck because to have a stake was to have a piece of the success when it came. An example of its usage is, "Joe has a $50 stake in the Jets game." Another phrase with similar meaning is to, "have a horse in the race." If one has a figurative horse in the race, they have a stake in the race/wager/sporting event.