That means the item is checked out to someone else.
If you need the book before it is due back, you can request it via ILLiad, our Interlibrary Loan service, and we should be able to get a copy for you from another library.
Or,
you can place a hold online for the item, call the library, or come in person. A hold will flag the item so that when it is returned it will be held at the Information Desk for you and you will receive an email letting you know it has arrived, and you then have 10 days to pick up the item before it is returned to the shelves.