What is a black hole? Can humans travel inside a black hole?
A black hole is a region of space from which nothing, even light can escape.
The Schwarzschild solution to the General Theory of Relativity predicted that if a massive body is compressed below a certain radius it will distort spacetime such that not even light can escape it.
The term black hole was given to describe such a region. Although we have never directly observed a black hole they are believed to exist because there are objects in space which as so small and massive they can only be black holes.
It is theoretically possible to enter a black hole, though it would be impossible to get out or even send a message out.
Black holes can be formed when a large star collapses at the end of its life and explodes as a supernova. The star's core, if massive enough, will collapse into a black hole. It would be impossible to enter such a black hole alive as when you get close the difference in gravity between your head and feet would tear you apart.
If the black hole is larger, then it would be possible to cross the event horizon, which is the boundary of the black hole, alive. The problem is then that you would fall into the central singularity of the black hole. This is a point of infinite density where everything entering the black hole ultimately goes.