What is the difference between "Reserve" and "Show interest in" a book?

You can reserve a book when you are guaranteed to borrow it at a later date. You will
see a queue number for books you have reserved.

It is not guaranteed that you will get to borrow a book that you have shown interest in. The library can keep track of which books patrons want, and they can use the expression of interest as a purchasing aid when buying new e-books and e-audiobooks.

There are several reasons why a book may be unavailable for borrowing. For example, your library may have run out of licenses for the book in question. In that case, they will have to purchase the book again to make it available for you to borrow.