In order to circulate digital content, libraries must abide by publisher lending policies. One of these lending policies is called the One-Copy, One-User Model. Under this model, libraries agree to pay for each digital copy they own just like the hard copies they circulate, so only one person has access to one copy of a title at a time. This means that if you want to borrow a One-Copy, One-User title that's already checked out, you have to put it on hold and wait for it to become available.
If you'd like to borrow a title that has a long waiting list, let your library know. They may be able to purchase more copies of in-demand titles.