What does Churchill mean by an "Iron Curtain having fallen across

While Churchill equals frequently credited with getting arisen this passage " metal blind, " he may, ironically plenty, get gone this period from counting Schwerin von Krosigk, that foreign minister of Germany in the last days of the war, who, the Times reported, had warned in a radio broadcast a few days before VE Day, " in the East the iron curtain behind which, unseen by the eyes of the world, the work of destruction goes on, is moving steadily forward."