On a roller coaster, we see the cars lifted to the high point on the ride. This converts the kinetic energy used to lift the cars into potential energy. When the cars are dumped over that initial apex, the potential energy is converted back into kinetic energy. As those cars continue along the track, conversions of kinetic energy to potential energy will occur at each rise along the course, and there will be conversions from potential energy back into kinetic energy at the drops. This continues until the end of the ride, where most of the potential energy that was initially put in has been lost in friction, and only a bit of braking is needed to stop the cars.