A ball bounces back when dropped on a floor because of the conservation of energy.  The ball’s potential energy is converted to kinetic energy as it accelerates towards the floor.  When it hits the floor, some of this energy is converted into sound and heat energy, and the ball bounces back with the remaining energy.  This time, the ball’s kinetic energy is converted back into potential energy.

Different surfaces absorb different amounts of energy when the ball hits the surface.  A hard concrete floor absorbs less energy, allowing the ball to keep most of its kinetic energy to bounce back higher.  A soft floor, such as a sand pit or carpet, will absorb more of the ball’s kinetic energy, resulting in the ball bouncing lower off the floor.