The black box, also known as the flight recorder, is a very important device that can be found in every commercial plane. It records a lot of data during the flight, for example altitude, speed, direction and many other things. It can also record sounds, including radio communication as well as any announcements and background noise.
You might be wondering why? It's simple: In the event of an accident, the black box can give information about what happened on board and how the crash occurred. That is very important to be able to learn from what happened and avoid another accident in the future.