Energy of Repose is a naturally occurring and automated buff available to players who are level 15 and above. This buff increases all experience obtained by your character by 40% and increases item drop rate by 10%.
You must log out of the game for a period of time to gain the buff. The percentage of experience you will gain upon logging in is based on how long you were logged out. Logging out in a studio or other home will increase the rate that you regain energy of repose. In Oriel, if you stay inside a circle of Street Band NPCs, it will recharge up to 15%, as well as heal your hp and mana.
The icon is displayed in the top-left or top-right area of your User Interface (depending on your settings), and you can hover over it for additional information.