When a vehicle is driving behind you at night with their headlights on, they may be causing a glare that makes it difficult for you to see what's going on around you. This distraction can be dangerous for both you and other vehicles around you. You can flip the switch on your mirror that will darken your mirror, but that darkness is often too extreme. Having a vehicle with an auto-dimming rearview mirror can make a big difference in your visibility.
Your auto-dimming mirror has two pieces of glass that contain an electrochromic gel in between them. That gel has the ability to change thanks to a low level of electric current that can be provided when your vehicle senses that there's an issue with glare around you. The change can go back to clear if it is daytime or the glare is no longer around.