GOF design pattern

In GOF design pattern we studied "Make a system independent to its realization"  what does this mean?  how will we implement this phrase in our software design?

Creational design patterns are design patterns that deal with object creation mechanisms, trying to create objects in a manner suitable to the situation. The term "realization" means the instantiation of creation of objects. In creational design patterns, the creation or instantiation of objects is kept independent of the system. It means that system is not concerned with when and how the objects will be created since these are created using a pattern.

You can find more about creational patterns from the link below:

http://en.wikibooks.org/wiki/C++_Programming/Code/Design_Patterns/Creational_Patterns