Artist's rendition of a black hole | Image: Ute Kraus
A black hole in space is a location in which the pull of gravity is so forceful that even light can not get out. Matter compacted into a small space renders gravity's pull so strong. One circumstance in which this can occur is when a star is dying. Without light, black holes are invisible to the eye; scientists locate them with specially equipped telescopes.
The linked article lists some interesting facts about this phenomenon in space. For example,
7. Time Slows Down Around It
"If you’ve seen Interstellar, you’ll know what happens when you travel near a black hole; time slows down. What is incorrect about the film is that the time dilation would not be quite that extreme.
Time dilation is ultimately affected by gravity, the stronger the gravity, the stronger the time dilation. Also, time only slows down once you get near the black hole, once you pass the event horizon, time would stop."