A superpower is a country that is simply much more powerful than the average country.  There is no exact line that separates a major power from a superpower, but the idea is that a superpower is on a completely different level of power than the average country is.

During the Cold War, there were two superpowers in the world.  These were the Soviet Union and the United States.  They had militaries that were so much bigger than those of any other country that they were considered to be superpowers.  These were the superpowers that were involved in the Cuban Missile Crisis.  The Soviets put missiles in Cuba and the United States pressured them to remove the missiles.