What does this option do?
Do you mean this feature?
Yes. But what does that last check box do?
Below is straight from Glide’s documentation, but I played with it a bit and I’m not entirely sure either. My original though was there was a difference between the sign in screen background and the loading screen background if you are already signed in and the app is loading. I couldn’t see a difference. The background image showed on the loading screen weather or not the checkbox was checked.
You can also enable the setting to use your background image in your app’s loading screen.