Potential energy depends on the height Ep= m*g*h. The more height the system has, more potential energy it owns. Meanwhile it starts to move, the energy transforms from potential (it's going down) to kinetic (it's gaining speed)
Ek = 1/2 * m * v^2
When the roller coaster reaches the ground, all the potential energy has turned into kinetic energy. As it starts going up again, once more the kinetic energy starts to transform into potential energy, due to the system is stoping and gaining height again.
