If your character enters a room and you notice a creature or NPC appearing out of nowhere, you have just entered a trigger. A trigger is an invisible area placed on floors, walls and even some objects, which instructs the game to take some type of action. In the above example, the trigger spawned in an encounter for your character.
Triggers are used to set off traps, spawn encounters and a few more different things. Triggers are setup to cover a specfic place inside a module area and there are different types of trigger, such as:
Encounter Trigger - This type will spawn in whatever type of creature(s), at the location you tell it to. You could, for example, spawn in an Ogre behind the PC party or a group of Skeleton Warriors in the next room.
Generic Trigger - These are used to spawn in objects or a more specific encounter, i.e., something you've edited from the standard encounter pallette and is now part of the custom pallete.
Trap Trigger - These are used for two purposes;
1) To set off the ol' spike in the floor trap. Well, any type of trap can be controlled by this trigger.
2) Secret trap doors. In this case, the trigger is checking your PC to determine if the PC has detected the hidden door or not. It also governs the range and DC of the check.
Sound Trigger - This is used to start audio play back. If you move to a spot on the map and start to hear birds chirping or the sounds of battle, you may have activated this type of trigger.
The best way to understand triggers, is to build a module area and place different types of triggers inside it, then play your module.
Experiment with triggers, enjoy their usefulness and have fun.