A black hole is an object so massive that not even light can escape its gravitational pull. The more mass an object has, the greater its gravitational attraction for other objects. For example, one weighs more on Earth than on the Moon because Earth has more mass and its pull on our bodies is larger.
Black holes are very compact and yet contain phenomenal amounts of mass.
Supermassive black holes are thought be a million to a billion times more massive than our Sun.