Problem with Oil
 

Hello,

I have mercedes-benz 170 CDI turbo diesel 1999, 180 000 km. Im very satisfied with my car, because its small, comfortable and very speed. But I have a little problem (I hope its little).
The oil is flows from a unknow component of my engine. This component is on the left side near filler of oil tank in diesel engine. Please see attachment.
On the picture is red color indicate place where is oil flowing. Then it flows down by pipe.
I have written about this problem to section A Class.
Reply to my problem was "Crankcase vent."
Could somebody explain me, what is the main function of crankase vent., please. Its a big problem for engine? How can I repair it?
Thank you very much.