Its the plastic pipe that connects the rocker cover to the air intake ducting. As the engine bashes up and down the air pressure of the engine parts moving has to go somewhere, so the oily air is fed into the intake. In old cars there used to be a mesh filter inline that used to get bunged up and when the engine went some the engine air pressure used to blow the dipstick up, force oil down the bores or out of any weak gasket. Sometimes the gunge depends on the types of journeys you do. Short, cold engine runs produce Oil/ condensation gunge, longer runs evaporates off any condensation.
Or sometimes if the air intake has been hacked off to make way for a K&N or something people have been known to block off the breather or just leave it open, which shoves out lots of oily smelling air.