Backlight panel or the back of the LCD panel is contaminated by moisture or dust. Notorious problem for LED backlight iMacs or cinema displays. Many customers in china actually protested to apple. Which is definitely on the news because protests are rare in china. for some reason....
You have to actually take apart the LCD assembly to clean it, which is an extremely difficult job. If you still have warranty, take it back to apple, they will replace the LCD assembly.
If you really decide to do the repair yourself. Prepare some extra aluminum foil tape and foam tape to try to seal it better when putting it back together. Also do not screw the LCD frame too tight, which can actually cause the frame to bend and allow leaks