The Settings button leads you to the Sky View Settings screen where you can specify what you want to see in the sky map. The button brings you back to compass view. The number indicates the size of the field of vision. Roughly speaking, Civil twilight is when it's getting dark somewhat and you should switch on the lights of your car. Nautical twilight is when it gets dark enough for the brightest stars to be visible in the sky, which was important for nautical purposes, hence the name.
Astronomical twilight is when it gets dark enough to observe even the faintest objects.
Now you have completed your first steps with Planetarium. To learn more about the capabilities of the program, please read the other chapters of the manual.