a cannonball hangs on a rope from an iron ring. a horse pulls the cannonball to the right side. when the cannonball is released it will move back and forth as diagrammed. it would do that forever except that the movement of the rope in the ring and rubbing in other places causes friction, and the friction takes away a little energy all the time. if we ignore the losses due to friction, then the energy provided by the horse is given to the cannonball as potential energy. (it has energy because it is up high and can fall down.) as the cannonball swings down it gains more and more speed, so the nearer the bottom it gets the faster it is going and the harder it would hit you if you stood in front of it. then it slows down as its kinetic energy is changed back into potential energy. "kinetic energy" just means the energy something has because it is moving. "potential energy" just means the energy something has because it is in some higher position than something else.