When your engine is running massive pressures build up inside the combustion chamber (this is the camber between the top of the piston and the head where the spark happens) as a result, some of this pressure gets past the piston rings and down into the crankcase (the bottom of the engine)
This pressure is bad, it can push oil up past the rings on the engine as its breathing in on each cylinder, to get rid of it you need the engine to be able to breathe out to atmosphere, when it does so though, it breathes a mixture of oil and air, what you need to do is catch the oil and return it to the sump, and let the air out.

Massively over simplified, but thats the short answer basically.