libby is an app that connects to your local library (through your library card #) and allows you to check out audiobooks and e-books. it works a lot like a physical library in that if they're not available, you can't check out book right away. you place holds on the books you want and they can take weeks or months until you can borrow them.
i used to hate that, but if you have many holds (up to 12) then you can read/listen to them as they become available and by the time you're done, the next is likely ready to be borrowed. you can borrow them for 21 days at a time.
there are other similar apps that offer similar things, some where you can borrow them right away, but libby has the best collection of books, including most new releases. i think libby blows audible out of the water, in that once you use libby, paying for a subscription and buying audiobooks seems absurd.