When I say sometimes, I mean most of the time the default Cocos2d-JS classes do not let you do everything you need to do with them.

Although this might seem like a limit of Cocos2d-JS, it's one of its best features. You are provided with a basic set of classes you can extend the way you need to, meaning you can add new capabilities.

So, what does it really mean to extend a class? Imagine a real-world example: you're entering a bike shop and buying a mountain bike. Your mountain bike is a class; with this class, you can do everything you can actually do with a mountain bike, namely pedal and steer.

Unfortunately, you are a bit lazy and don't want to pedal all the time, so you buy a little motor and add it to your mountain bike. Now, you can still do everything you usually did with your bike, but you can also rest your legs, turn on the motor, and let it pedal on your behalf.

You just extended the mountain bike, created a motorized mountain bike...