Plex is a media server and personal library that's able to organize the media stored on your computer, like videos, TV shows, music, and stream that content to iOS devices, the Apple TV, and other set-top boxes.
When you install Plex and let it organize your content, it'll add artwork to movies, TV shows, and music, along with information like cast members, plot summaries, and Rotten Tomatoes ratings. On the Apple TV, this turns into a nicely organized media library that largely resembles iTunes or Netflix.
Plex has been available for a long time on set-top devices like the Chromecast and the Fire TV, but it has not previously been available on older versions of the Apple TV in an official capacity. Those who have jailbroken an Apple TV in the past have been able to unofficially install the Plex software.
The Plex app for the Apple TV can be downloaded from the tvOS App Store. The app is free to download, and unlike the iOS version does not require a $4.99 in-app purchase or Plex Pass subscription to unlock functionality. Apple TV and iOS App Store apps are universal, so if you've already downloaded the app for iOS, it can be found in the Purchases section of the tvOS App Store. [Direct Link]