That's the evaporative emissions canister. It collects gasoline vapors from your fuel tank so a lot of parked cars wont produce a gasoline stench on a hot day. It can be taken off and the line capped. On the hottest day of the year, with a full tank of gas, you might see the cap pop off and your gastank will hiss at you when you open it. Other than that, it is another item under the hood taking up space and little more.