Are you serious? You searched the internet and couldn't find what one does, how bad are your search skills
In a nut shell they catch/seperate oil vapours so the oil doesn't build up in your intake system.
You can have them venting to atmosphere or plumbed back in, the later one being the legal option.
Crank cases are pressurised and therefore need to vent somewhere, from factory this venting goes back into the intake and on an engine with a few km's on it the oil vapours can get quite substational which leads to many negative effects. People install catch cans to stop this oil depositing throughout their engine, instead being caught elsewhere (the catch can). It's a mod most of us most likely have.
Hope that helps. Seriously just do some more searching, catch cans aren't that complicated.