Thanks to built-in sensors, Android is capable of many things. In this case, it's the built-in gyroscope, which can determine the device's orientation and act on movements. So the screen can adjust its orientation automatically depending on how you hold the device, from landscape to portrait and vice-versa -- even if you hold your device head down.
This usually works fine -- but there are cases where the screen orientation does not auto-adjust when you want it to -- or it auto-adjusts when you do not want it to (think e.g. of reading an eBook while lying on your bed). If you run into one of these cases, this is the tag to be used.
See also: orientation for general issues with the gyroscope sensor