Positive Crankcase Ventilation.

piston rings even when brand new have a certian amount of blowby. Which pressurizes the cank case. Without a venting system this pressure will deplete total HP output and cause oil leaks.

So hooked to your carb or intake on the vacuum side is generally a 3/8 rubber hose attached to the PVC valve on one of the valve covers. The pvc unit it's self is a simple one ball valve. When the steel ball feels carb vacuum it opens and allows manifold vacuum to vent the crank case.

Motors actually see a hp increase up to about 20 inches of vacuum. High crankcase vacuum levels are only attained by aftermarket vacuum pump motors or dry sump systems