It is not known yet what the Source exactly is. The Source seems to be the birth of all energy.
The compounding nature of energy means that The Source, which is infinitely large, does not run out of energy. The Source provides raw energy, which is somewhat unusable, though can be manipulated to create spirits, which refine the energy into Cosmic Commons much like crude oil is refined into more usable form for combustion. It takes Cosmic Might to harvest this raw energy.