Think of potential energy like you are a marble. If a marble is at the peak of the hill, it's potential energy is ENORMOUS because as gravity pulls it downward it will increase in speed. Halfways down the peak is about the same potential energy as the peak, although slightly less. At the end of the climb, we can assume the landscape flattens out. This means there's very little pull to make it change at all. It will continue with the speed gained from the downhill trajectory until something in its environment stops it (such as friction). When a marble is at the bottom of a large hill, the potential energy is low. It takes a lot of force to try and get up the hill, losing lots of energy
