Credits to the team that developed PlexConnect, specially @baa @roidy and @f00b4r (the core team), but also to the many others that contributed to enhance PlexConnect.
ATV2's and ATV3's were created by Apple to stream media from Apple's catalog and from user's iTunes networks. Users willing to stream from PMS (Plex Media Server) to ATV2/3 need to use PlexConnect.
Apple no longer produces (or sells) these ATV models, and consequently PlexConnect is no longer developed. Many users decided to replace their ATV2/3 with more recent boxes (or SmartTV's), but there is still a large community using PlexConnect.
PlexConnect is an "unusual" Plex client. It can't be installed in AppleTV's (except jailbroken ones). It must be installed in another machine (usually the one running PMS). One of the ATV2/3 applications must then be "hijacked" so that it "speaks" with PlexConnect instead of the Apple servers.
This "hijacking" process has 3 steps:
1. Restore the ATV to factory settings (loading the latest available firmware)
2. Load a security certificate in the ATV (which is generated during the PlexConnect installation)
3. Modify the DNS entry in the ATV (so that it poins to the IP of the machine running PlexConnect)
After applying the above "hack", whenever you select the "hijacked app" in the ATV, PlexConnect will capture the request and present a series of screens through which the user has access to PMS. A single PlexConnect install can drive multiple ATV's, but all of them must be in the same subnet, and you need to install the same "hack" in all of them. This also implies that ATV's cannot be "remote" to PlexConnect (unless there is a private VPN).
PlexConnect was developed using Python 2.7, therefore it can run in any machine with Python 2.7+ installed. Easy installers exist for the following platforms:
1. Synology NAS
2. Mac OSX
3. Docker (Intel 64-bit)
Other platforms require a manual installation process. Not a complex one, but maybe too "techie" for some users. You can find more details here
PlexConnect is not supported by Plex Inc., similar to all other third-party Plex clients.