A protien skimmer it a type of cylindrical equiptment that is used to remove waste from the aquarium that normal surface skimming cannot do!
What it does exactly, is it takes water from the tank and it sucks it up in the main camber and from there, tiny microbubbles are formed causing them to be pushed up the skimmer towards the collection cup (sits on top) and when the bubbles are released/popped a thin foam/frothy material is what's left behind (It is normally dark in color- the darker the waste is, the more junk has been taken out of the tank. It's a good sign to get a really dark color).
The jbj nano cube is very tiny and I'm not really sure if a skimmer can fit in it it not. You could check out the nano skimmer (I think it's made from current) and see if you can fit one of those. You might what to see if anyone else has any ideas!
P.S.- If I've said some of the info that pertains the the skimmer and is not quite stated right please correct it. And there may be someone else that can describe it to you alittle better than I can!
I hope this helps alittle!