System windows are the parts of the screen where the system is drawing
either non-interactive (in the case of the status bar) or interactive
(in the case of the navigation bar) content.Most of the time, your app won’t need to draw under the status bar or
the navigation bar, but if you do: you need to make sure interactive
elements (like buttons) aren’t hidden underneath them. That’s what the
default behavior of the android:fitsSystemWindows=“true” attribute
gives you: it sets the padding of the View to ensure the contents
don’t overlay the system windows.
https://medium.com/google-developers/why-would-i-want-to-fitssystemwindows-4e26d9ce1eec