As a roller coaster approaches a large hill, the potential energy of the car increases. As you approach the top of the hill, this is where the potential energy reaches the maximum level. Going down the hill, while increasing your speed, the potential energy transforms into kinetic energy. At the very bottom of the hill is where the kinetic energy is at it's max. While making a loop, right before the loop the cart has kinetic energy that partly transforms into potential energy, almost equal when the cart is completely upside down. Coming down from the loop, the speed increases again, resulting into that potential energy to turn into kinetic energy once again.