The over/under represents the combined amount of runs/points/goals scored by both teams during the game. Bets can be placed on whether the total amount of points will be over or under the amount offered.

NBA Example: if the over/under for an NBA game was 225 points, a bet placed on "Under 225 points" would win if both teams combined to score below 225 points, and would lose if both teams scored more. 

If the Raptors beat the Warriors 114-110, the "Under 225 points" bettors would win since the total number of points scored was 224.
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Tennis Example: if the over/under for an tennis game match was 21.5 games, a bet placed on "Under 21.5" would win if the players finished the match in less than 21.5 games and would win if the players finished the match after more than 21.5 games. 

If Federer beat Nadal 7-6, 1-6, 6-3, 6-4, the "Under 21.5" bettors would lose, since the total number of games played was 39.