I hope you know about escape velocity of a planet or heavenly body. It (Escape Velocity) is the minimum speed with which an object has to be projected from the surface of a planet/star so that it never returns (escapes). magnitide of escape velocity depends upon mass and radius of a planet/star, assuming it to be a spherical body. For example escape velocity from the surface of the earth is 11.2 km/s. Note that it is km/s not km/h :). Now, if mass and radius of a planet/star is such that its escape velocity comes out to be greater than speed of light (c = 3 * 10^5 km/s) then even light is not be able to escape from its surface. You might know that we can see an object only if either light is emmited from an object or reflected from it. Object with escape velocity grater than c (velocity of light) will neither emit nor reflect light which makes it the BLACK HOLE.