Libraries purchase a limited number of digital copies of books from publishers, and only one person can check out an eBook at a time.
If a book is checked out by someone else, you'll see an option to ‘place a hold’ on the book. Once you place a hold on a book, it will be added to the ‘holds’ section of your account. When the book becomes available, you'll get a notification email.
Learn more about the OverDrive service here: