A real-life example should make it clear. Three long-time friends happen to be in the same city on business, and they plan to have dinner together, and then go and see a movie, just to relive old times.
One of them is going to take a car, and they happen to be the nearest to the restaurant. So, they arrive quickly and wait at the venue. The other buddies are taking a train and a bus, respectively, so everyone waits for the other(s) to arrive.
Once everyone arrives, the dinner can begin. This agreement of waiting upon other buddies to join, and then only begin the fun, is a barrier:
After the dinner is over, and as people eat at different rates, not everyone finishes at the same time. So, again, they wait (barrier), and then, once everyone is done, they go to the movie.
A barrier is very much like a countdown latch, with the only difference being that...