Every massive object in space has an escape velocity. Escape velocity is the minimum velocity an object must have in order to escape the gravitational strength of a particular planet or any large body in space. The earths escape velocity is about 11.2 km/s. This means that an object must travel 11.2 km/s to escape its orbit around the earth. Reaching this velocity is a very big challenge when dealing with space travel. The more mass a body has, the more gravitational attraction it has; therefore, the escape velocity becomes faster.
A black hole's escape velocity is so high that nothing can escape its gravitational pull; not even light. That is why its call a "black" hole: there is no light coming out of it so you can't see in it. A black holes escape velocity must be greater than 300,000 km/s (the speed of light). How can in object have this must mass to generate this much gravitation? When a star reaches the end of its life, some of them collapse all the way down to a single point, maintaining it's mass. This means that it becomes infinitely dense. Our sun does not have the potential to eventually become a black hole because it is too small. Stars that become black holes are 20 times the mass of the sun.