I read it long back,if I am correct, the book is comparing between inheritance and composition.
When you extend a class and make a subclass then you get the behavior of super class and the behavior is static (you can not change the behavior at runtime). But if you compose then the behavior you get with composed reference could be dynamic.
SCJP 1.5 94%.
The greatest glory in living lies not in never falling, but in rising every time we fall.