it means 'kalidoscope copy wheel eye'. It is the updated version of the sharingan(copy wheel eye). The mangekyo sharingan posesses a few power full abilities like tsukiyomi,amaterasu and susanoo. these powerful abilities can be used at the cost of some of the user's eyesight until the user becomes blind. That why they say 'a kalidoscope(mangekyo sharingan user) will eventaully lose its(his/her) light(sight)'
So far, only 5 people have it. Namely inuza uchiha(deceased killed by madara), madara uchiha(aka tobi killed inuza uchiha for mangekyo sharingan eyeballs to gain eternal mangekyo sharingan and imortality),kakashi(got his sharingan from obito uchiha who died shortly after)(supossedly deceased, if so killed by pein's deva/nakara path),itachi uchiha(deceased, terminal desease and sasuke uchiha)(got his mangekyo by killing shisui uchiha his best friend) and sasuke uchiha(first had itachi's sharingan in him but after hearing the truth from madara, he awakened his own mangekyo sharingan).
There will probably be no mor people with the mangekyo because the rest of the uchiha are all dead.