What does stereo illumination on a stereo mean?

Illumination is the ability to lighten the screen or darken the screen with the other dash lights on the vehicle. normally you will have two orange wires coming from the cars wiring harness one with a white stripe one without. The orange/white wire will make the stereo dim or brighten with the dash lights. the solid orange wire will dim/brighten the stereo with the headlights. so if it is night you will have the headlights on, when you turn the headlights on the stereo dims so the light from the stereo will not blind you at night. while the headlights are off the stereo will be bright so the stereo light will be visible in the daytime