Original: Mitte encompasses Berlin's historic core and includes some of the most important tourist sites of Berlin (like the Museum Island, Brandenburg Gate, Unter den Linden and the Reichstag among others), most of which were in former East Berlin.
Simplified: Some of the most important tourist sites of Berlin (like the Pergamon Museum, Brandenburg Gate, Charité and the Reichstag), are all in Mitte, and most were in East Berlin.