An event horizon is the boundary surrounding a black hole beyond which nothing can escape, not even light. It marks the point where the gravitational pull becomes so strong that escape velocity exceeds the speed of light. Once an object crosses this boundary, it cannot return or be observed from the outside. Event horizons are crucial in understanding the nature of black holes, which are formed from the remnants of massive stars after they collapse. The concept is also important in the study of general relativity, as it challenges our understanding of space, time, and the fundamental laws of physics.