Energy is the main Campaign resource.
You need Energy to complete missions in Campaign mode or to pass them instantly with Memory Spheres.
Energy restores itself over time.
Energy limit increases with every new level. You start the game with 30 energy limit, you gain 1 additional unit from levels 5-65 and 2 additional units from levels 66-80. In other words, you end up with 120 energy when reaching level 80.