EVAP Canister
WARNING: This page is about a different car, the 2009 GMC Canyon and 2009 Chevrolet Colorado. However, it is still accessible from the selected car via links, so may be relevant.
The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.