Energon is emnation of Primus, and lifeblood of all Transformers. It flows from Cybertron's core outward, spreading life across the planet. But, a tainted and corruptive equivalent called Dark Energon was infused into the core to halt the flow of pure Energon.
During the Great War, much Energon was harvested and stored in cubical units. These energon cubes were used by Autobots and Decepticons alike to regain lost energy.