Let's recap. Flash movies, comprising timelines and whatever they contain, have a particular set of objects that are accessible to ActionScript.
Objects are broadly defined as functional units consisting of some combination of data (properties) and programming instructions (methods).
The way in which objects are instantiated from a classand, therefore, gain the properties and methods of that classis known as inheritance. Using the prototype property of an object, you can add or change the methods and properties available to any new or existing objects that inherit from that class.
The ability to use inheritance to alter existing ActionScript objects means that you can expand upon common functionality and change the behavior of those built-in objects. It also allows for greater flexibility when working with your own custom objects.