Glycogen is basically many strings of glucose molecules attached to one another.
Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body.
Glycogen functions as one of two forms of long-term energy reserves, with the other form being triglyceride stores in adipose tissue (i.e., body fat). In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. 
The main function of muscle glycogen is to provide glucose units for glycolysis in the exercising muscle. Thus the glucose units produced from breakdown of muscle glycogen stay in the muscle providing fuel for ATP production. However, the main function of liver glycogen is to provide glucose to maintain blood glucose levels during exercise.
As long as glycogen stores hold out, the liver does a remarkable job of maintaining blood glucose levels, preventing exercise induced hypoglycemia.
The major function of muscle glycogen is to supply glucose units for muscle energetics. The major function of liver glycogen is to maintain blood glucose levels, which is critical as during exercise muscle extracts glucose from the blood for fuel.