What is the name of bar that contains hardware back button on android emulator and how to show it when it disappeared?

I am making a small app with React-Native.Then when I navigate to the next page, the android bottom bar (contains hardware back button) disappears.What is the name of this bar, and how to show it when it disappears?

This is an inactive state.

This is an active state.

How to show this bar? And how do we call this bar?