An oil catch can takes in blow-by fumes / oil from your engine and deposits it in a 'can' allowing air to be deposited into your air intake instead of oil. This reduces fumes that are normally burned out through your engine.
Here is the technical detail:
I bought this for my truck but the valves are too small. This will do good in a car. Has an adjustable breather and external reader so you can see how much oil it catches. The pressure meter moved but did not seem to be accurate, I did not get much out of it. For a truck you need to have one with a much larger inlet. I am just putting this here for reference as I sold this one.