Toward the end of each half of a soccer match, the fourth official on the sideline hoists an illuminated sign over his or her head that displays a number. This magic number indicates how much injury time (or "stoppage time," depending on your vernacular) the head referee has decided to add to the 45 minute half. But how is this calculated?
What constitutes whether something is worthy of stoppage time varies from league to league, but in FIFA competitions like the World Cup, the referee is tasked with keeping track of how much time has been spent on substitutions, injuries and injury treatment, time-wasting, and "any other cause." Refs usually keep track of the duration of these instances on their wristwatches and then, with a few minutes remaining in regulation, inform the fourth official of how much time they have decided on so they can let the players, coaches, and fans know via the aforementioned board. (Referees can add more time in the midst of stoppage time on the fly if something occurs that calls for it.)
The origins of stoppage time are up for debate. One widely cited but possibly apocryphal story is from England in 1891. Aston Villa were up 1-0 against Stoke City when a Villa player deliberately punted the ball out of the stadium so the clock would run out by the time Stoke City were able to retrieve the wayward ball. Henceforth, referees were allowed to add on time to balance out any future shenanigans.
So, if your favorite team has ever lost because of a goal in the sixth minute of stoppage time, maybe blame Aston Villa.