PFE is the capacity for energy output.
Lets use a AAA battery as an example; if you have a flashlight with one battery you have a certain amount of energy that can be used over time. Connect another battery and you'll either get more energy as in a brighter light or the same light over a longer period of time.
And that energy comes from the ATP cycle (we'll skip the biochemistry) which can be considered a finite amount until you eat more food.
Muscles use the ATP cycle to generate an amount of power for the size of the muscle. The bigger the muscle the more power can be made available or the higher the PFE.
Note that Muscles do NOT store 'Potential Energy (PE)'