Muscle uses ATP to contract. You go into the gym and use your muscles. You need more ATP. We actually don't have high stores of ATP in the body. Luckily for us, our muscle stores something called creatine. Creatine can bind to spare phosphates. For the replenishment, creatine can hold onto some of those phosphates. So it creates creatine phosphate which is a backup store that you use so that you can just wait trained for a little bit longer.