What is a black hole?

A place where gravity has become so strong that nothing can climb back out of it, not even light. One forms when the core of a very large star collapses at the end of its life.

The boundary is called the event horizon, and crossing it is one-way. That is also why a black hole looks black: no light leaves it to reach your eye. It is not a hole in the way a hole in the ground is. It is a place where the pull never lets go.