A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a massive star is dying.
Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help to find black holes. Special tools can see how stars that are very close to black holes act differently than other stars.