The Mangekyou Sharingan (literally Kaleidoscope Copy Wheel Eye in Japanese) is the 4th and highest level of the Uchiha clan's Sharingan (a technique from the wonderful anime, "Naruto"). Aquiring it signifies complete mastery of the Sharingan, which allows usage to techniques such as Amaterasu and Tsukiyomi. The secret of the Mangekyou Sharingan is that only those from the Uchiha clan who have achieved the 3rd level of Sharingan (3 teardrops in both eyes), and have killed their best friend, may use it. So far in the Naurto manga, Uchiha Itachi is the only one who has been revealed that has achieved such a level.
"If you open your eyes to the truth, including me, the number of people who can handle/deal with the Mangekyou Sharingan will have become three." - Uchiha Itachi to Uchiha Sasuke