Chance Miller, writing for 9 to 5 Mac:

The app is rather basic in terms of features and you must be a Prime member in order to use it. Upon downloading it, you’ll be given a code and asked to head to Amazon’s website on your phone or computer to sign in. From there, the app will ask you to create a four-digit pin number for completing purchases.

My theory is that Amazon is testing the waters to see how many of their users have Apple TVs and whether it would be worth developing a video app for. Or maybe that’s just wishful thinking.