Entry is static nested interface, inside interface Map. This is almost like a static-nested class inside a class.
Consider the following code:
You can reference this via Outer.Inner, and even instantiate an object of an anonymous inner class, for example:
Simply put, a static nested interface behaves similarly to a static nested class - except it is an interface. (Technically it is not an "inner"-interface, just as static nested classes are not "inner"-classes).