Answer:
There’s only a certain amount you can compress some amount of mass before every molecular force is overcome, and the object collapses under the force of its own gravity. This “point of no return,” the radius past which everything collapses, is the “Schwarzchild radius,” and when the mass becomes a black hole that becomes the radius of the black hole’s event horizon. Once you pass the event horizon, it’s game over: not even light can escape!