To establish potential energy, you must first establish the conditions of the situation for energy. For example, the height or distance of the ends of the track from the floor is different than the height or distance of the ends of the track from the seat of the chairs. Therefore, you must establish the conditions of height or distance. The marble held at the end of the track with the established conditions of height or distance has the potential to MOVE the length of that established distance or height if a force, namely that of gravity, were to release the potential energy of the marble. Work has been done.
Work is the force times the distance.
The marble gains energy by traveling down the slanted track. The higher the marble's starting point, the greater the energy it has at the bottom. On top of each hill the amount of energy the marble has is called potential energy. When you place the marble in the valley, or at the bottom of the big U, it doesn't go anywhere. However, when it rolls into this valley from the top of the track, it keeps traveling.