Black holes are prisons of light. They are both metaphor and physical entity, mute commentary on what is known, unknown, and unknowable. Well-studied but poorly understood, like a virus.
What happens gravitationally if you squeeze the mass of an object to a point? This was Karl Schwarzschild’s question while stationed at the Russian front of the First World War. You get a point of no volume and infinite density a singularity. It would be surrounded by a region where nothing, not even light, could escape. This boundary became known as the event horizon because no event within the boundary could be observed from outside.