A flag is just a 'thing' that is either 1 or 0. on or off. Now you can do all funny things with them suchs as using them in conditions (false, true, time since flag activation, etc). Example: flag 1 turns on when a convoy enter a trigger zone. Next trigger has condition time since flag 1 20 seconds and the action is the activation of an ambush. This will make the ambush appear 20 seconds after the unit entered the trigger zone. Thats a very basic application but you can imagine the possibilities. Advanced waypoint actions for example can also be controlled by flags by using the 'is user flag' start or stop condition