Why do we need the decorator in the decorator design pattern?

Presuming I have a class named A, and I want to use the decorator design pattern. Correct me if I'm wrong, but for that to work , we'll need to create a decorator class, say ADecorator, which will hold a reference to an A instance, and all the other decorators will extend this to add functionality.

I don't understand why do we have to create a decorator class, instead of using an A instance?