Potential energy is stored energy.
Water behind a dam has potential energy that can be converted to kinetic energy. In human body, chemical energy in food is converted to chemical energy in ATP and then converted to mechanical energy of muscle contraction. Energy cannot be changed from one form into another without a loss of usable energy. When muscles convert chemical energy in ATP to mechanical energy, some is lost as heat. Energy conversions result in heat and therefore the entropy of the universe is always increasing.