If you have a class that you either wrote yourself, or is part of the system, you can create a new class or even a variable, based on the first class, and you can allow the new class to "inherit" all the properties, functions, etc., of the original class. (This is called a "child class".) Once you create the child class and it has inherited all the features of the "parent" class, you can add additional properties, functions, etc.
I have simplified this to make it easier for you to understand. There's more to it, but this is the basics.