I can't find an explanation for it anywhere.

The footprint is called the Book Passport, and using it you can view the swapping history of a book.  Often, there isn't much there, because the history begins when a book is first posted into the system.  But a book that you receive from PBS and repost back into the system starts to develop a traceable "trail" and you can follow that trail by using the footprint icon.  Kind of along the idea of BookCrossing, you can keep watch of where a specific book has traveled.

Pretty nifty feature, especially interesting for the books that have been reposted several times.

I'm adding the Help Center information, since there is a little more explanation there:

introduced 10/8/2007.

The Book Passport allows you to see the swapping history of any book on your Bookshelf, Transaction Archive or TBR Pile. 

• Just click the   ("footprint") symbol on the listing for the book to see the information.
• Each time a book is marked received, it counts as a "passport stamp" for the book.
• The total number of miles a book has traveled appears at the top of the Passport Screen.

It's probably my very poor vision, but I had thought those were just odd questionmarks! So after reading this thread, I went to go look. Then I uncovered an oddity I would like someone to explain. Why do some books have them and some do not? Only about half mine have the passport, but most of the ones which do have only the one stamp, from whomever to me. So why don't all the books have them? Everything in the TBR list came from PBS and should have at least the one stamp.