A black hole is region in spacetime from which nothing can scape, it has a frontier which is known has event horizon, after this point not even light can get out. How does a blakc hole born? that's easy to explain, when a huge star finishes its gas, nothing can stops the collapsation, so it becames an undimensional point in spacetime with a volume equal cero and an inmense density, as a result we have a singularity, this is the hipotetical nucleo of the blackhole, this how they born.

so be ware if you fall down in a phenomena of this kind you'll never return.