The Berlin Wall was a wall which was started in Berlin but divided the whole of West Germany from East Germany from the 1960's to the 1980's. When the Berlin wall came down that was the beginning of the end of the USSR empire.
East Germans knew there was more freedom, better living standards and better life generally in the West so many East Germans wanted to move over. That wasn't so easy after the wall was built. Armed soldiers prevented East Germans fleeing to West Germany and countless East Germans were shot trying to cross over the wall.
The West Germans called the wall, "Die Schandemauer" (the wall of shame).