PCV stands for Positive Crankcase Ventilation.
When an engine is running, some of the combustion mix in the cylinders gets past the rings and ends up in the crankcase. That'* called 'blow-by'. In the olden days on gas tractors they used to have a little pipe sticking down from the engine that let these gases out. In the interest of cleaner air, years ago they incorporated a system where a breather tube lets air into the crankcase, and then another tube goes from the crankcase through the PCV valve back to the intake manifold. As gases escape past the rings and into the crankcase, they are mixed with the fresh air from the breather tube and sucked back into the engine and reburned.
The valve is there so the gases only go one way. If it gets plugged, it can cause problems with your idle, and it can cause oil consumption and in really bad cases, oil can be pushed back up the breather tube into the air cleaner, tho I don't know where these engines get the fresh air from.
They are easy to check, basically you shake it and listen for the ball rattling inside. If no rattle, it'* plugged. Someone said they spray WD40 up into it also, don't know about that, they're so cheap you could just replace it.
The valves are located in different places depending on the year. You'll have to get a Haynes manual and check it out.
1996 Olds LS, 198,900 miles: RIP...