That’s the number selected for the total points of the game. If the Kansas City Chiefs are playing the Los Angeles Chargers and the over/under is listed at 55 that means a bettor can bet the total score of the game will either be under 55 points, or over 55 points. So if the bettor bets under 55 and the final score is Kansas City Chiefs 31, Los Angeles Chargers 21, he, or she, wins the bet. If another bettor wagered on the over 55, he, or she would have lost the bet. Had the score been Kansas City Chiefs 31, Los Angeles Chargers 24 the over/under would have been a push and all the bettors would have broke even.