Energy in Clash Mini
Energy is what units use to cast their special abilities. Most units in the game have a special ability that they can use. When viewing a unit's information, these abilities will have the word "Super" at the beginning of them. In order for these abilities to be used, the unit must get their energy bar to maximum. Once it's full, the unit's next attack will use the ability instead.
Your units get energy by hitting other units or getting hit by enemy units. In addition, many units start with different amounts of energy, so be sure to check out the stats of each unit to see how much they start with.