USB tuners on Android TV allow users to watch over-the-air channels and even record them locally to watch them later. The feature should be included in the firmware for Android TVs running version 7.0 and up, but when the NVIDIA Shield TV received the update last month USB tuner support was notably not available yet. This feature will come in a future update, but individuals who are interested now are able to use a custom system image.
The guide comes from user Linux4all on the GeForce forums. With a downloaded boot image, you can then flash that onto your Shield TV. When you reboot, you then get full access to USB tuners in Live Channels. Based on the screenshots the feature seems to work very well. It has been tested with a handful of Hauppauge USB tuners.
The custom kernel is available on GitHub as well, so others can contribute to the codebase or port to another device.