Although I don't have that, I think that is the charcoal canister. The charcoal canister is used to control evaporative emissions. It's basically a way of venting your gas tank and the fuel vapors other than venting to the atmosphere. It's used to reduce hydrocarbon emissions. Usually, when you fill up, gasoline overflows and goes in there... don't fill your tank up to the brim to prevent gasoline in entering the vent to the canister....