Someone's gonna come along and explain this MUCH better than I. But, basically, for emissions, etc., there is a hose going from the engine back to the intake in order to recover and burn any unburned fuel and gases. Oil can travel in that line. Oil can then get into the intake and turbo which can drastically reduce their life. A catch can has a hose running to it from the engine while the intake's hose fitting is plugged. The catch can has a breather element (think air filter) and a can to "catch" the oil. You empty the catch can every few thousand miles. Again, my understanding only!