Liquid crystal display screens are much better than the cathode ray tube screens they replaced. LCD screens use less power and do not burn out as fast, but they do need periodic conditioning to maintain maximum resolution.
LCD screens contain a liquid (touch one gently and you can see this) that has a particular reaction to electricity. A small electrical charge applied to a liquid crystal changes its transparency in a small local area. This property -- plus a lighting element behind the screen -- makes an excellent computer screen.
Unfortunately, if the same area is illuminated the same way for hours on end it can become "stuck." This means that the area in question retains its transparency characteristics when the electricity is removed. These areas are usually on tool bars or other areas that remain fixed for hours while the computer is being used.
You can run a program that will rejuvenate your LCD screen. These "LCD conditioning" programs come standard on many computers with LCD screens. If you do not have an LCD conditioning program you can download one for free from the Internet. Experts recommend you run the program twice a year.