Your body burns the food you eat in much the same way that a fire burns some kind of fuel, be it wood, gas, oil, etc. The fire rapidly combines atmospheric oxygen with molecules in the fuel to make carbon dioxide and releases the energy in the chemical bonds of the fuel as heat and light. On the other hand, your body combines the food with oxygen that you breathe in a controlled manner (also producing CO2), harvesting the energy of those chemical bonds to power those turbines that produce ATP. ATP is just a molecule that holds a lot of energy in its chemical bonds. It's the "energy currency" of life, providing the overwhelming majority of the energy required by other molecules as they do their jobs in your body (think of your muscles contracting). ATP is the convenient, universal way to move around energy in living systems. Can't do none of this shit without breathing and eating.