Glycogen stores refer to the reserves of glycogen, a complex carbohydrate, that are stored in the liver and muscles. Glycogen is produced from glucose through a process called glycogenesis and serves as an energy reserve for the body. When the body needs immediate energy, such as during exercise or fasting, it breaks down glycogen into glucose through a process called glycogenolysis. The liver can also convert excess glucose into glycogen through a process called gluconeogenesis and store it for future use.