The LED is the backlight for the LCD. Traditional LCDs use floresscent bulbs (which contain mercury), the new LCDs use LED (which still contain "dangerous" materials, but they can easily be broken down to safe minerals, it also uses less energy). Some white LED bulbs will yellow with use, this was occuring in a percentage of the first gen MBs and MBPs. Make sure you get AppleCare just in case you get some yellowing.
TEG