You won't learn if you don't ask
It "catches" the oil in the vapour that accumulates in the Cam covers - these usually vent back to the intake so the oil vapour gets run through the engine and burnt off before being expelled through the zorst, downside is the oil can mess up your intakes, the catch can has a baffle in it and is placed in the line the vapour hits said baffle and the oil drips into the bottom of the can to be disposed of later.
If that's not enough They look