Vent valve and canister with drag

That's the Evaporative Emission Control System. It traps fuel vapors that escape from the gas tank, throttle body and intake manifold. It stores the vapors in a charcoal cannister and when the engine is running it releases the vapors back into the engine to be burned. You could have poor idling, stalling or a strong smell of fuel if this system malfunctions. If the check engine light came on it probably needs to be changed but you might want to get a second opinion.