Since you know how video rentals work, this is easy to understand. You rent an audio CD and receive it, you listen to it as much or as many times as you'd like until you feel like you want to send it back. You don't pay shipping. When they receive your rented CD back you can order another one. You can do this as many times during the month as the back-forth-listening allows according to your subscription. You can choose to rent one at a time, or more. So the subscriptions are based on how many you want to take out simultaneously, to listen to and send back and order again as many times as the month can fit in.
This starts with the first month at half price. The second month (should you not cancel) would be at full price.
Audio books are the adventure in the spoken word. They are the exact texts of the entire book without abridging.