The auto lights function uses a sensor to determine if it is dark enough to warrant turning on the headlights automatically for you. If it detects that it is dark enough, like when you go into a tunnel, it puts the headlights on. As it has put the headlights on it also dims the dashboard lights to their night time setting. Sometimes I also find I think it should have put the lights on as it isn't very bright, but obviously the sensor isn't triggered, so I have to manually put them on. This usually occurs on overcast days or coming up to dusk.
An alternative is to leave the headlights set to on. This way you have day time running lights on and when it is starting to get dark or you enter a tunnel your headlights are on. When it is dark your headlights are on as well.
When you leave the car it will turn off the lights as you have removed the ignition key.
