potential energy

Potential energy is stored energy: energy an object has because of where it is or what condition it is in, rather than because it is moving. A book on a shelf, water behind a dam, a drawn bowstring and a stretched rubber band all hold energy that is not doing anything yet.

Height is the usual source. Raise an object and you have done work against gravity, and that work is now stored; the higher it is, or the heavier it is, the more there is. Release it and the store converts into kinetic energy as it falls. A spring stores energy by being deformed instead — the more you compress or stretch it, the more it holds — and food and fuel carry chemical potential energy released when they react or burn.

The pair with kinetic energy is the whole subject of the questions. At the top of a swing or a roller coaster's first hill, the car is momentarily still, so its kinetic energy is at its lowest while its potential energy is at its highest; at the bottom the two have traded places. Total energy is conserved in that exchange — only the form changes.