Among the features of JDK 11, we have several hotspots (changes at the bytecode level). One of these hotspots is known as JEP 181, or nest-based access control (nests).
Basically, the nest term defines a new access control context that allows classes that are logically part of the same code entity, but which are compiled with distinct class files, to access each other's private members without the need for compilers to insert accessibility-broadening bridge methods (Java documentation).
So, in other words, nests allow nested classes to be compiled to different class files that belong to the same enclosing class. These are then allowed to access each other's private classes without the use of synthetic/bridge methods.