That's the crankcase breather, part of the PCV system. Normally, air goes into the crankcase thru that hose, & gets sucked out of the crankcase thru the PCV valve.

At WOT when there's not much vacuum in the intake manifold, blowby can overwhelm the PCV valve. Then the flow reverses in that breather hose & air blows from the crankcase back into the intake.

Breather filters are for that hose. You disconnect it from the intake, plug the nipple at the intake, & put the breather filter on the crankcase nipple. That should be illegal if you get smog checks where you live.