What is it?

I believe what you are describing is the carbon canister unit. Part of the EVAP system, it's the same on a lot of Fords. The engine's computer controls the purge solenoid which draws fuel vapors back into the engine. The carbon filter is supposed to last the life of the car, but the EVAP canister itself may need to be serviced if it develops a leak, and you'll see a trouble code if that ever happens.