This is when you discover that the launcher is the home screen, wallpaper, buttons and sometimes the vehicle logo that you see on the android headunit screen. The launcher allows you to see the speed, current track, change volume, select tracks, start the GPS mapping or any app you have on your headunit. Changing your launcher can open up your android headunit to a lot more than is was as standard.
The launcher app is the home screen, the desktop with the icons. The screen you see when the headunit has booted up. It the main interface that the driver has to use to operate the head unit.