The furnace pressure switch shuts down the furnace if the unit has improper air pressure caused by venting issues. Potential causes of pressure change that signal a problem include leaks, restrictions in the intake air vent, restrictions in the combustion air vent or a problem in the draft inducer motor.
The switch is near the draft inducer motor, which is responsible for moving combustion byproducts out of the house through the vents. When the pressure isn't correct, those potentially dangerous byproducts cannot properly move out of the furnace through the flue vents. The pressure switch is a safety feature that stops the furnace in this situation so the combustion exhaust doesn't linger in the house.