Dubbed PlexConnect, the hack was developed by Plex users Baa, roidy, and f00b4r. It involves running a program that pretends to be Apple's Trailers app on a computer connected to the Apple TV and changing a network setting on the Apple TV. Those two acts combined essentially fool the Apple TV into accepting Plex content without having to "jailbreak," or hack the device to run unauthorized code.
The workaround gives tech savvy Apple TV owners a way to have access to yet another media service. Plex allows users to organize their videos, photos, and music, but the software has until now been unavailable on non-jailbroken Apple TVs.
Jailbreaking an Apple TV or any iOS device can be a risky affair, as users will at the very least void the warranty, and at worst could render their device non-functional. Jailbreaking iOS-powered devices like the Apple TV and iPhone allows users to run code not approved by Apple, opening up new features and capabilities.
As it is a workaround it is possible that future Apple TV firmware updates could disable PlexConnect. Given that the Plex community developed the hack, though, any such feature loss would likely result in a quick adaptation of code to reenable PlexConnect.
A version of Plex for the iPhone and iPad is available in the iTunes App Store. It allows users to stream and navigate content from their and their friends' Plex servers over Wi-Fi or a cellular connection.