Event Horizon

 Straight to the point,

Horizon = a limit to what we can observe.

Event Horizon = A limit after which whatever events happen will not be observable.

Every object has a gravity that attracts other objects. To escape its gravitational hold, you have to move at a specific escape velocity. For Earth, it is 11.2 km/s. For Jupiter, it is 59.5 km/s. You get it, the more mass the body has, the more its gravity, and the more its escape velocity. 

Black holes are basically bodies of unimaginable mass that is concentrated in an unimaginably small area. It's infinitely dense. The closer you get to it, the more velocity you need to escape. 

The point after which the escape velocity is more than the speed of light is the event horizon. Hence the popular "not even light escapes" and the nomenclature of "black" hole. So, when light does not come back, you can't see what events happen after that point, e-v-e-n-t h-o-r-i-z-o-n.

This is why black holes are mysterious and ominous, because the singularity, where the laws of physics break down, is essentially unobservable. To observe it, we need something travelling at a speed much much faster than light, something that can achieve the escape velocity, which for all we know now, is impossible.