The event horizon is a key concept in the study of black holes and general relativity. It is the boundary surrounding a black hole beyond which no light or information can escape, due to the immense gravitational pull of the black hole. Once an object or particle crosses the event horizon, it is essentially trapped within the black hole and cannot escape. The event horizon plays a crucial role in understanding the behavior and properties of black holes, as well as the nature of spacetime in the presence of strong gravitational fields.