This is a term used in sports, such as baseball, to describe the number of games a team must win in order to secure a place in the playoffs. It also refers to how many games a team must win to clinch a title, be in the Division, League, or World Series title. The most common usage is the former, and is calculated by using the total number of games in the season, adding one, and then subtracting the sum of the total number of games won by the team seeking the win, and the losses of the team right behind them in the standings. Fans and broadcasters count down this magic number towards the end of the season.
This phrase is used in most sports that have a playoff structure at the end of the regular season. The number of games a team must win to secure a spot in those playoffs is heavily dependent on the number of games in the season. For example, the NFL season is only 16 games, while Major League Baseball plays 162 games. Each game has larger playoff implications in the NFL than in baseball.