Megali Idea is a decision for expansionist Greece. It requires that Greece owns all of Thessalia, East Macedonia and West Macedonia. Those are all the provinces that Greece has cores on, but is owned by The Ottoman Empire at the beginning of the game. On top of that Greece must have invented National Fraternity, an invention which is unlocked with the tech Nationalism and Imperialism.
The decision gives cores on Ottoman owned Thrace, Hudavendigar and Aydin (modern Bulgaria and western Turkey). On top of that it gives 20 prestige and removes 5 infamy from Greece.