A CountDownLatch is a construct that a thread waits on while other threads count down on the latch until it reaches zero.

A CyclicBarrier is a reusable construct where a group of threads waits together until all of the threads arrive. At that point, the barrier is broken and an action can optionally be taken.

We can think of this like a group of friends. Every time they plan to eat at a restaurant they decide a common point where they can meet. They wait for each other there, and only when everyone arrives can they go to the restaurant to eat together.

https://www.baeldung.com/java-cyclicbarrier-countdownlatch