What do we mean by the event horizon of a black hole?

The event horizon is an imaginary surface surrounding a black hole, where the escape velocity is equal to the speed of light. Therefore, nothing within that horizon can escape due to the attraction of a large gravitational field

Explanation:

No particle that falls within the event horizon comes out again, since a speed greater than light would be needed, and so far no particle can reach it. In this way, there is no way to look inside the event horizon. For this reason, black holes have no visible characteristics.