At the middle of a black hole, there is a gravitational center called a singularity. It is impossible to see into it because the gravity prevents any light escaping. Around the tiny singularity, there is a large area where light which would normally pass by gets sucked in as well. The edge of this area is called the event horizon. The area beyond the event horizon is the black hole. The gravity of the black hole gets weaker at a distance. The event horizon is the place farthest away from the middle where the gravity is still strong enough to trap light.