Energon is the Transformers' main source of energy. For them it is fuel, food and the most valuable resource all at once. Almost every big story in the franchise revolves around Energon in some way: someone steals it, someone guards it, someone searches for new sources because Cybertron has run dry.
What it looks like
Energon is usually shown as a glowing pink or blue substance — liquid, crystalline or in cubes. In the classic cartoon the Decepticons make 'Energon cubes' by converting Earth's energy sources: power plants, oil, even sunlight. In Transformers: Prime Energon is mined as crystals from underground deposits.
Why they fight over it
Whoever controls Energon controls the army. That's why many classic episodes follow a simple pattern: the Decepticons try to steal Earth's energy and the Autobots stop them. A shortage of Energon is also why both factions ended up so far from home.
Varieties
Different versions add special forms: synthetic Energon, unstable Energon or, in Prime, Dark Energon — 'the blood of Unicron', able to reanimate the dead. The Archive has a separate article about it.
