It is exactly what Mike's post shows, a crankcase breather. It's an EPA thing to take the gases from combustion leak past the rings (known as "blow by") to end up inside the crankcase. The gases used to just vented into the atmosphere but for "environmental protection" it was deemed by the government that these gases must be contained via an enclosed system.
These gases solidify into a type of film and it's the reason we get the crud encrusted throttle bodies and reduced performance from the build-up unless it's cleaned periodically.

The small filter catches these on their way out. An even better solution is a "catch can" which can be purchased online or at any auto parts store...

 

So it actually has air flow coming out not actually a vacuum. Right?

 

Air flows toward the air box. The air box creates a sizable low-pressure area when the engine is inhaling. That low-pressure condition pulls fumes from the crankcase.