Potential energy is energy based on an objects position with respect to the Earth. If you take a book and raise it over the floor over your head, you have given the book potential energy. The book will start to accelerate towards the floor the instant you release it, converting the potential energy to kinetic energy, which is the energy of things in motion. But back to your question, if the object is already on the ground, the potential energy would be equal to zero, because it is not raised above the Earth in any form or fashion. For the object to have any energy at all, energy must be loaded into the system for the system to work. Back to the example of the book, as long as you allow the book to lie on the floor, it will not move, because it has no energy. If you pick the book up, you are storing energy into the book. This energy is called potential energy because it has a chance, or potential, to change into another form of energy, kinetic energy.