top level nested class means static inner class ... as you may know that to accessing static members JVM don't need objects . so we can access this static inner clsss without the object ( instance ) of outer class , we can access them directly by class name .
hope it will help . if want more details then many experts are here to help you out .
"Top-level nested class" is an old term for a static nested class. Calling this "top-level" was a bad idea in the first place, needlessly confusing (how can something be both nested and top-level?), and when Sun released the Java Language Specification 2nd edition several years ago, they removed this odious bit of terminology. (Thank you, Sun.) That is also why the tutorial Edwin referenced does not mention the term. Unfortunately many books still exist which use the term, either because they're old, or the author(s) didn't realize the olde term had been removed from the language. So, any time you see the term "top-level nested class", just pretend they said "static nested class" or "static member class" instead.