By default, colliders are set with the isTrigger property unchecked, meaning that the physics system treats them as solid objects. However, in some cases, you'll want to be able to pass through a Collider component without it stopping your GameObject. This is where triggers come in. With isTrigger checked, a GameObject can pass through it, but the Collider will send out the OnTriggerEnter, OnTriggerExit, and OnTriggerStay notifications.
Triggers are most useful when you need to detect when a GameObject enters a certain area or passes a certain point. We'll use this to set up the areas around our enemies; if the player walks into the trigger zone, the enemies will be alerted, and, later on, attack the player. For now, you're going to focus just on the enemy logic in the following challenge.