It basically means that the only part of the library which you're going to use you should rely upon is it's API (Application programming interface) and that you shouldn't base your application on the concrete implementation of the library.
eg. Supposed you have a library that gives you a stack. The class gives you a couple of methods. Let's say push, pop, isempty and top. You should write your application relying only on these. One way to violate this would be to peek inside and find out that the stack is implemented using an array of some kind so that if you pop from an empty stack, you'd get some kind of Index exception and to then catch this rather than to rely on the isempty method which the class provides. The former approach would fail if the library provider switched from using an array to using some kind of list while the latter would still work assuming that the provider kept his API still working.
