A Game Trigger is a switch that is turned on once a certain point is passed or a specific objective has been completed.
Probably one of the most common usages of a trigger in any Call of Duty game is events. Game events often occur after completing an objective and command teammates and even enemies to do something else, such as move up to the next area or take cover behind a specific object.
In Call of Duty 4: Modern Warfare, and Call of Duty: Modern Warfare 2, a common usage of a trigger is in the playable character's squad. More often than not, the rest of the teammates will not move up to the next area unless the player move past a certain point with them.
Disrupting a trigger can lead to an unexplained death, such as at the end of Heart of the Reich in Call of Duty: World at War, if the player tries to kill the German soldier that burns Chernov with an M2 Flamethrower.