This indicates a fuel vapor leak. The evap system controls fuel vapor containment. It holds vapors in a special canister which is emptied when the computer commands a solenoid to open a valve and allow the vapors to be sucked into a runniong engine. Fuel vapor vented to atmoshere is considerded pollution by the EPA.
Most time it is simple a loose gas cap. Pull it off and put is back on tightly. Have the code cleared and see if it returns.
It could also be a bad cap, wrong cap, a small leak in the hoses and tubing associated with the system, or problems with the canister and its valves.