Behaviors are just a fancy way of saying things or interactions that will happen in the game. Breaking down these actions or reactions in this way helps to componentize how we think our game will work. Stopping and thinking about this from the very beginning means we won't get too many surprises later on. (There are always surprises after a good night's sleep.)
For example, behaviors can take the following forms:
Attacking another entity
Taking damage
Collecting the loot, which could be money or items
Teleporting to another land
Behaviors on classes should only affect the class that it is defined on. If you are going to affect another class's attributes, it should be through another behavior on that class.