Interfaces and inner classes are more sophisticated concepts than what you’ll find in many OOP languages; for example, there’s nothing like them in C++. Together, they solve the same problem that C++ attempts to solve with its multiple inheritance (MI) feature. However, MI in C++ turns out to be rather difficult to use, whereas Java interfaces and inner classes are, by comparison, much more accessible.
Although the features themselves are reasonably straightforward, the use of these features is a design issue, much the same as polymorphism. Over time, you’ll become better at recognizing situations where you should use an interface, or an inner class, or both. But at this point in this book, you should at least be comfortable with the syntax and semantics. As you see these language features in use, you’ll eventually internalize them.