It meters air from the intake tube to behind the throttle body, through the crankcase (well, through the valve covers, but same deal). This means you don't need a crankcase vent and you can burn whatever comes out.
They contain moving parts -- mainly a check valve -- and can wear out or get plugged. They are VERY cheap -- often around $5 -- and take a minute or two to change, so quite a lot of people change them routinely as part of a tune-up, whether they need them or not.
If the check valve sticks, it can function as an air bypass, and can make the idle difficult to control. Or it can plug, leading to pressure leaks somewhere in the crankcase (valve cover gaskets or grommets are likely).
