There are two flavors of relativity, special and general. Special relativity says that movement at a constant velocity in a straight line is equivalent to standing still. From the premise, and the observed fact that the speed of light is always the same, it is straightforward to prove that nothing can go faster than the speed of light. No matter how fast you travel, you MUST see light traveling away from you at the speed of light. So light must be traveling faster than you are.
Light is the fastest thing there is, and nothing else travels as fast as light.
Now let's apply this fact to black holes. The harder you throw the ball, the higher the ball will go. You will have given it escape velocity. The escape velocity, obviously depends on the amount of gravity, which depends on mass and the distance. If light can't go fast enough to escape, then nothing else can.