What is a black box on a plane?

What is a black box on a plane?
What a black box is forA black box preserves the records of what happened in the event of a disaster. The box is made of a nearly indestructible material so that even if the plane is destroyed, the box can be found and retrieved. The box contains recordings of the status of the plane, including measurements of altitude, air pressure, as well as mechanical status. They don't just make the plane out of the stuff because it would be too heavy to fly.