This indicates that a book is already checked out and that another person has requested the book once it's returned. You can add yourself to the waiting list for the book by using the "Request" button near the top left of the catalog record.
If you need the book right away, you may want to check to see if there is another copy available in the MOBIUS system that you might be able to get more quickly than you would by adding yourself to the waiting list for SLU's copy. If you need any help, please ask a librarian.