It is the main resource that you need to continue entering PVE rooms like Nephthys dungeons, World Conquest dungeons, Tower of Dawn stages and Lost Island. Entering a dungeon will consume your energy but a portion of it will be returned if you failed to clear it. Every 4 minutes you'll gain 1 energy. Other modes also grant free energies such as Battle of Honor and Dimensional Breakthrough, hour attendance in the laboratory gives 50 energy twice a day and Guild attendances reward will also give free energies as a reward depending on guild attendee. Every time you gain a level you energy restores to max. There's an item called Energy potion that can also be used to fill your energy to max.