Backlight dimming

Can anyone tell me what the backlight dimming does for a lcd tv.

In the costumer service menu you can turn it off,but when you turn the tv off,next time it will be on again.

I found it very annoying when there is a talk program,there is constant a difference in light.

Backlight dimming is a trick to increase the potential contrast in an LCD screen. Older technology LCD displays would just have a fluorescent lamp to provide the light. The LCD panels would block the light to produce a picture. There would almost always be some light leakage. Then manufacturers started using LEDs for backlights which allowed them to be shut down during dark scenes, increasing the contrast level. Then they switched to zone backlighting - where they could keep some areas light while others are dark. The next change was to switch to edge lighting. It wasn't a better performing solution, it just allowed them to make the displays thinner. That's where we are today.