A GameObject inside the Prefab instance that you opened Prefab Mode through.
If, to open Prefab Mode, you clicked the angled right arrow next to a Prefab instance root, this GameObject is that root. If you opened Prefab Mode via a keyboard shortcut, then this GameObject is the one you had selected when you used the keyboard shortcut. The GameObject is null if you did not open the Prefab Stage via a Prefab instance.