It is positive crankcase ventilation. Its purpose is to reduce emissions, and reduce sludge.
When the engine has high vacuum some air is routed through the crankcase and then sucked into the intake manifold. These air is metered (measured by the mass air flow) so that all air going into intake is accounted for an the proper fuel is delivered.
This helps reduce moisture from accumulating in the crankcase. It also keeps the engine from smoking a little bit like the pre PCV cars did.
Emmisions are reduced because that smoke or vapor has some hydrocarbons in it that is now sucked into the intake and burned.
