A sports betting spread is to create an active market for both sides of a game rather than a win or lose outcome. A spread is a range of outcomes that are available for both teams and the bet is whether the outcome will be above or below the spread.
For example; in the NFL if the New England Patriots were to play the Jacksonville Jaguars and the Patriots had the best record in the NFL at that time and the Jaguars had the worst, the spread betting option would come into play so that customers can bet on both sides of the game.
Therefore, the Patriots would have a spread of -10 and the Jaguars would have a spread of +10, simply because of the difference of quality in the sides. This therefore gives customers an option to take the Jaguars at a realistic spread, which makes bets much more interesting and less one sided. So, if the game was to end 21-12 to the Patriots and you had +10 Jaguars, the Jaguars would win the game by 1 point and you would win your bet.
This format is on the rise in the sports betting world and is definitely a good format to use when a game includes a clear favourite and an underdog.