Can someone explain to me what that is? 

Box-O-Books lets you ship books for less money, and no credits are needed.

Why use Box-O-Books?

• It saves you money!  Sending multiple books in one shipment costs much less in postage than sending single-book shipments.
• It saves you credits!  When you don't have any credits, you can still get books!
• This service is especially useful to new members; they can swap whole boxes of books with other Boxers, thereby replenishing their library while waiting for their newly Posted books to be requested and earn them credits.
• Box-O-Books is the only way that PBS members are permitted to swap "books-for-books" without using credits.

Box-O-Books is a paid subscription feature at PBS

•  Enrollment in Box-O-Books  is $8.00 per year
  • Box-O-Books can be purchased in the PBS Kiosk.
• There is no limit on the number of Box-O-Books swaps you can conduct in a year

To expand on Mary's info. Basically you pay the fee to join. THen  you can make book for book deals with other people who have paid to become boxers.  If you want to check out what other boxers have before deciding to join you can. Just go to the Box of Books section and search for some books you may be interested in trading.  This is a good way to get rid of heavily posted books. I don't recommend it if you have a lot of books on your shelf that you have no interest in reading. Because a lot of the BOB offers you get will be from people with shelves loaded with those same types of books.  There are a lot of boxers with varied shelves so you might do all right anyway. I just know this from experience.  I had a lot of little series romances on my shelf when I joined BOB. (It's on hold right now).  I got them cheap and then didn't care for them. So I posted them here and every BOB offer I got was for guess what?  People with nothing but those little series romances on their shelves.  So I was turning down BOB offers constantly until I ran a deal and got rid of them.