One should be able to easily access the most important features of the phone when holding it with one hand; it supposes touching the screen with the thumb only. That doesn’t necessarily mean that all the screen surface and features must be accessible by the thumb (given the planned 5.5″ screen size, that would not be physically possible), but that the lower area is preferable to access the most useful phone features by default. Those features would include answering an audio or video call, reviewing notifications, unlocking the phone, accessing the home screen, requesting a search or launching the most frequently used applications.
The red area is the area of effect that the thumb will be able to reach to
Source: Designing the Mobile Experience with Convergence in Mind – Purism