What was referred to as the "iron curtain?"
The main way that the term “iron curtain” has been used in history is to refer to the boundary between the communist bloc and the West during the Cold War.
The Cold War was a conflict between the Soviet Union and its communist allies (or satellite states) and the United States and its allies, which opposed communism. Each side wanted to dominate the world and both sides feared the other. During the Cold War, the Soviet Union controlled all of Eastern Europe and all of Western Europe was more or less allied with the United States.
The Soviets did not want their people to come in contact with the West or to be able to travel freely to the West. They knew that people who came in contact with the West would probably prefer Western ways because the communists did not have a strong economy and they did not allow their people much in the way of personal freedom. The communists also knew that people who were allowed to travel to the West would often simply stay there because they did not want to live under communism. They saw this, for example, in the number of people who fled from East Berlin into West Berlin while it was still easy to do so. As this link tells us, something like three million people went from East Germany to West Germany just between 1958 and 1961.
Because the Soviets knew these things, they created the “iron curtain.” This was the boundary between the two sides which separated the communist world from the free world. In Berlin, the boundary took the form of the Berlin Wall. In other places, it took the form of double fences to prevent anyone from crossing. The term “iron curtain” emphasized the idea that the Soviets wanted to completely block their people from the West, making it impossible for them to get out or really even to “see” what was outside.