As Angels, Demons function on Essence provided by the God-Machine, the ephemeral stuff of the Shadow that also feeds spirits. Once an Angel falls and is cut off from the God-Machine's supply, he can see Aether emanating from the engines of their Creator. Human physics understands the concept as entropy: the fact that in any reaction a certain amount of energy is lost as waste heat. Demons know that the same principle applies to the occult physics of the God-Machine and its Infrastructure, and have found ways of exploiting that form of waste heat for themselves.
Demons use Aether to repair and maintain their Covers, use Exploits and enhance their physical capabilities, in their demonic form as well as under Cover. Aether is stored in a Demon's Primum and has to be learned before it can be wielded efficiently, as a freshly fallen Demon has little understanding of this new source of energy and how to properly utilize it.