Triggers are used to check for event occurence during a mission. You might check that the player is above a certain altitude, or has landed at a key airport, or that a certain amount of time has elapsed.
When a trigger is active, it will fire whenever the appropriate conditions are met.
When a trigger is deactivated it is not evaluating its conditions to decide whether it should fire its actions.
In multiplayer mode, triggers with the IsGlobal property set to True will affect all other players.
If a trigger does not contain this property, or the property is set to False, the trigger will affect only the local player.