The Iron Curtain Speech was a speech given by Winston Churchill on March 5, 1946 that was meant to encourage the United States to take a more aggressive stance towards communism.
By this time, Churchill was no longer the Prime Minister of England. He was visiting the United States as a private (though very important) person. He felt that the US and the other Western allies were not taking the Soviet Union seriously enough as a threat. Therefore, he gave this speech in which he warned that the Soviet Union had put Eastern Europe behind an "iron curtain" and that it was planning to try to expand as far as it was allowed to.
This speech is a very important moment in the early parts of the Cold War.