ADP cycling supplies the energy needed to do work in a biological system, the thermodynamic process of transferring energy from one source to another. There are two types of energy: potential energy and kinetic energy. Potential energy can be thought of as stored energy, or usable energy that is available to do work. Kinetic energy is the energy of an object as a result of its motion. The significance of ATP is in its ability to store potential energy within the phosphate bonds. The energy stored between these bonds can then be transferred to do work. For example, the transfer of energy from ATP to the protein myosin causes a conformational change when connecting to actin during muscle contraction.
It takes multiple reactions between myosin and actin to effectively produce one muscle contraction, and, therefore, the availability of large amounts of ATP is required to produce each muscle contraction. For this reason, biological processes have evolved to produce efficient ways to replenish the potential energy of ATP from ADP.    
Breaking one of ATP's phosphorus bonds generates approximately 30.5 kilojoules per mole of ATP (7.3 kcal). ADP can be converted, or powered back to ATP through the process of releasing the chemical energy available in food; in humans, this is constantly performed via aerobic respiration in the mitochondria. Plants use photosynthetic pathways to convert and store energy from sunlight, also conversion of ADP to ATP. Animals use the energy released in the breakdown of glucose and other molecules to convert ADP to ATP, which can then be used to fuel necessary growth and cell maintenance.