ConcurrentNavigableMap is an interface in Java's java.util.concurrent package that extends the ConcurrentMap interface and provides additional methods to support the use of a navigable map in a concurrent environment. A NavigableMap is a type of SortedMap that allows for efficient navigation of the elements in the map based on their keys.

The ConcurrentNavigableMap interface provides methods to navigate the map and retrieve elements based on their position in the map, as well as methods to perform concurrent updates to the map. The interface includes methods to retrieve subsets of the map, such as subMap, headMap, and tailMap, which return views of the map that are themselves navigable maps.

The ConcurrentSkipListMap class is a concrete implementation of the ConcurrentNavigableMap interface in Java. It provides a scalable, concurrent implementation of a sorted map that uses a skip list data structure to maintain the elements in sorted order. A skip list is a probabilistic data structure that allows for efficient search and insertion of elements in a sorted sequence.

ConcurrentSkipListMap is useful in scenarios where a sorted map needs to be shared by multiple threads. It supports a high level of concurrency and provides efficient access to elements in the map, even in high-concurrency scenarios. It is particularly useful for implementing high-performance concurrent caches and other data structures where fast access to elements in a sorted order is required.