In your body you have a compound called ATP (adenosine tri-phosphate). Think of ATP as an energy containing compound. What is important to know about ATP is that the body can very quickly get energy from a ATP reaction. You have other sources of energy such as carbohydrates and fat - but they take longer to convert into a useable energy source. When you are doing an intense quick burst activity - such as lifting a weight or sprinting, your muscles must contract and need a quick source of energy. This immediate energy comes from ATP.  When your muscles use ATP for energy a chemical process happens where the ATP is broken down into two simpler chemicals ADP (adenosine di-phosphate) and inorganic phosphate. This process of ATP turning into ADP releases the energy which gives your muscles the ability to contract. Unfortunately, we do not have an endless supply of ATP. In fact, your muscles only contain enough ATP to last about 10-15 seconds at maximum exertion. In case you were wondering - no, the ADP can not be used to create more energy for your muscles.  Creatine that is stored in the muscles bonds with abundant phosphorus stores in the muscles and is converted into Creatine Phosphate (CP). CP is able to react with the ADP in your body and turn "useless" ADP back into useful energy - ATP. More ATP in your body means more fuel for your muscles.