Simply put, a CountDownLatch has a counter field, which you can decrement as we require. We can then use it to block a calling thread until it's been counted down to zero.

If we were doing some parallel processing, we could instantiate the CountDownLatch with the same value for the counter as a number of threads we want to work across. Then, we could just call countdown() after each thread finishes, guaranteeing that a dependent thread calling await() will block until the worker threads are finished.

https://www.baeldung.com/java-countdown-latch