A game-time decision in football occurs when there is uncertainty surrounding a player's ability to play in an upcoming game. The player generally has a 50-50 shot of playing, with an official decision not coming until moments before the game starts.
The game-time decision tag is often caused by injury. Players dealing with minor injuries may not be able to participate in regularly scheduled practices leading up to the game. They often use the warm-up period just before the game to determine whether or not they can play through the ailment, which is where the "game-time decision" designation comes from.
Game-time decisions make setting fantasy football lineups extremely difficult. Since managers are unsure of whether their player is going to play or not, they must choose to either bench or start the player and hope the right decision was made. If a player is benched and ends up playing, managers can end up losing valuable points that would've helped secure a win against the opponent. On the other hand, by starting a player that is labeled a game-time decision, the manager risks the player not performing well due to playing at less than 100% strength or not playing at all.
Game-time decisions are further complicated when players are scheduled to play in a Sunday or Monday night game. Most fantasy leagues require managers to set their lineups prior to the start of the Sunday afternoon slate of games beginning at 1 P.M. Given that managers are forced to rely on updates regarding injured players at least 12 hours before game-time, it becomes nearly impossible to be sure of whether or not a player listed as being a game-time decision is going to play or not.