"20/21 Back" is a term used when talking about a Vintage Star Wars Carded items.  Here's how to tell if your item is a 20/21 Back.  On the back of the cardboard (aka: Card) packaging the figures came in there is a picture of 20 or 21characters.  If your item has that, then you have a 20/21 Back.  They also realeased a card with a photo of 12 characters on the back.  If you have one of those then you have a 12 Back item.

Picture of a 20 Back: