It's just cleaning the inside of the barrel, unless they are talking about having the inside of a barrel treated with something.
A nice slick surface inside the barrel, without any smeg from previous paint makes a good surface to fire through. Brass is great for polishing and makes a great barrel, even with scratches and stuff. Just get a fluffy squegee or soft rag on a stick and get rubbing!
Often if you are playing with rentals, just giving the barrels a good clean can improve the performance of the gun by an immense amount.