PC is an abbreviation for phosphocreatine. PC, like ATP, is stored in the muscle cells, and when it is broken down, a large amount of energy is released. The energy released is coupled to the energy requirement necessary for the resynthesis of ATP.
The total muscular stores of both ATP and PC are very small. Thus, the amount of energy obtainable through this system is limited. In fact, if you were to run 100 meters as fast as you could, the phosphagen stores in the working muscles would probably be empty by the end of the sprint. However, the usefulness of the ATP-PC system lies in the rapid availability of energy rather than quantity. This is extremely important with respect to the kinds of physical activities that we are capable of performing.