a japanese way of describing a plural of ' stuff '. however, stuff is already plural, but it being a plural noun, if there are more than one item as part of ' stuff ' , then people will add an s.
The real stuffs is the good stuffs, meaning, you know you got the stuffs. When you have the stuffs, then you got the top quality, whether it be green or white, natural or man made, the finest stuffs.