Answer:
From the law of conservation of energy, energy cannot be created and cannot be destroyed but can change forms. So, total energy remains conserved. A body posses potential energy by virtue of its position or configuration. So, as the height changes, the potential energy changes. A body in motion possesses kinetic energy.
When the skateboarder goes up, it has both kinetic energy and potential energy. At the highest point, the instantaneous velocity is zero, so the kinetic energy is zero and skateboarder has maximum potential energy. Halfway down, it has both kinetic and potential energy. The potential energy decreases due to decreasing height which converts into more kinetic energy. At the lowest point, because height is zero, all the potential converts to kinetic energy.
If these is any friction present, some of the energy dissipates. But the total energy remains conserved always.