If a Library book has a 'hold' on it, it means that someone has reserved the item and the person the item is currently on loan to has received notice of a recall. If an item has more than one hold (sometimes, on the Library catalogue, you will see a message in the 'Status' box saying for example, 'DUE 20/12/2011 + 2 HOLDS'), this means that more than one person has placed a reservation on that item. As soon as any item is reserved by three different people, we receive an alert which means we review the item. When this happens, we often move the book to a shorter loan category and buy further copies.
For more information on reservations and recalls, visit: http://www.dur.ac.uk/library/using/borrowing/holds/