Energy of Repose is a buff available for players level 20+ that increases the amount of XP you gain when hunting. In order to receive the buff, you must rest or log out of the game for a period of time. The percentage you have upon logging in is based off of how long you rested.
The icon is displayed in the top-left area of your UI and you can hover over it for additional information.
The percentage can be different based on your current level:
Level 20-44 -- 40%
Level 45+ -- 30%