CountDownLatch is a class in Java's java.util.concurrent package that provides a way to synchronize threads and coordinate their execution by allowing one or more threads to wait until a set of operations being performed in other threads completes. A CountDownLatch works by allowing one or more threads to wait until a fixed number of operations, represented by a count, completes.

A CountDownLatch object is constructed with an initial count, which represents the number of operations that need to be completed before the waiting threads can proceed. Each operation is represented by a call to the countDown() method, which decrements the count by one. When the count reaches zero, the waiting threads are released, and they can proceed with their tasks.

The await() method is used by a thread to wait for the count to reach zero. When a thread calls await(), it blocks until the count reaches zero or until the thread is interrupted. The await(long timeout, TimeUnit unit) method can be used to specify a timeout period, after which the thread will resume execution even if the count has not reached zero.

CountDownLatch is useful for coordinating the execution of multiple threads and ensuring that one or more operations complete before other threads proceed with their tasks. It is commonly used in scenarios where multiple threads need to perform some initialization or setup work before the main application can start, or where multiple threads need to synchronize their execution to perform a complex operation. Examples include network servers that need to wait for all client connections to close before shutting down, and parallel processing applications that need to wait for all threads to complete their work before combining the results.