To start with, a black hole is not an actual hole in space. The most correct thing to do would be to speak of a black hole as a place where gravity is so strong that it pulls in all the matter around it; the gravity is so enormous that even light cannot escape this trap, and is also sucked in by a black hole. This is why they are called “black”--since they consume light, it is impossible to see black holes without special tools. Although the size of a black hole is not necessarily large (in fact, they can be rather small--in the scales of space, of course), due to the high concentration of matter in a relatively small place, its gravitation can be equivalent to dozens, or even hundreds of stars. Because of the exhaustion of hydrogen and helium fuel inside of a star, it cannot sustain its own size, so its huge mass causes such a star to “fall” into itself; the mass, however, does not vanish, so such a collapse results into the concentration of an enormous mass in a relatively small place.