Through the magic of science or optics or something, LCD screens are able to create white light by mixing other colors of light. This works fine, but not being satisfied with "fine," Sony has gone and added an entirely new pixel of pure whiteness, which can double a screen's brightness or make it twice as efficient, but not both at the same time.
If you stare at your monitor closely enough to give you permanent eye-strain, you'll notice that it's made up of arrays of red, green and blue pixels. Somehow, mixing red, green and blue can produce practically every color that humans can see, including yellow, which last time I checked was a primary color.* Making black is easy: you just turn off all the pixels. Making white is just as easy: turn all the pixels on. But, now you're using three pixels when all you really need is one single pixel of white.
Sony's new WhiteMagic LCD takes a conventional LCD display array (of red, green, and blue pixels) and adds a fourth, white pixel to the mix, effectively making the display RGBW. By using a clever algorithm to turn these white pixels off and on at the right times, the WhiteMagic display can either appear twice as bright as a conventional display, or it can maintain the same brightness while using half as much power. Otherwise, it looks and performs exactly the same a regular RGB LCD.
Obviously, having a screen that sips half the power at a given brightness would be a much appreciated upgrade to cell phones and laptops, just as having a screen that can be twice as bright at the same power would be great for using mobile electronics outdoors. Happily, this is no lab-bound research project, and Sony says that they'll be shipping the WhiteMagic tech as soon as October of this year.
*For the record, yellow is a primary color (along with magenta and cyan) when pigments are combined subtractively, as in paints or inkjet printers. When colors are combined additively (as with light), you only need red, green and blue.