That famous metaphor was used by Winston Churchill to describe the Soviet Union forming its infamous Eastern Block at the start of the Cold War.
Metaphorically, the Iron Curtain symbolized the ideological differences between communism and capitalism. The two sides were economically opposed to such an extent that there was almost a “wall” between them.
Physically, the curtain was the name given to the defensive emplacements between the borders of capitalist and communist countries. This border started in the north between the USSR and Finland, extending down across the borders of East and West Germany and finally between Bulgaria and Greece. Yugoslavia wasn’t really part of either the East or West block, so it’s position relative to the “curtain” is debatable. The Iron Curtain was also very visable in some places. The Berlin Wall is probably the most famous part of the Iron Curtain.
Churchhill wasn’t the inventor of the phrase. It actually appeared in print around the 1900’s, but he was the first to use the metaphor to describe the communist block countries.
In some places it was called other things. Between Russia and Alaska it was known as the Ice Curtain.