It's a vacuum chamber.
It's for your vacuum operated heat and air conditioning system
It contains a check valve and maintains a steady vacuum supply to the HVAC system.

 

To elaborate, your HVAC system uses vacuum diaphragms to operate the various flapper doors in the HVAC box that direct airflow to the different vents. This vacuum comes from manifold vacuum. At wide open throttle, manifold vacuum drops to zero, which would cause the flapper doors to resort to their default positions. This vacuum ball provides vacuum to hold the doors in the selected positions even when you step on the gas to go up a hill or pass a car. Depending on the options your car has, one of these reservoirs would also be used to operate a vacuum trunk release, since without it the trunk release would only work with the engine running.