What was the "Iron Curtain" that Winston Churchill referred to?

Winston Churchill's "Sinews of Peace" address of 5 March 1946, at Westminster College, used the term " iron curtain " in the context of Soviet-dominated Eastern Europe: The Iron Curtain as described by Churchill at Westminster College

In one of the most famous orations of the Cold War period, former British Prime Minister Winston Churchill condemns the Soviet Union's policies in Europe and declares, "From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the continent.