A superpower is a country that is one of the most powerful countries in the world. It is more powerful than a major power.
In the years following World War II, the United Nations was formed. The 5 countries that would later have nuclear bombs – those who would able to start a nuclear war – were all given permanent seats on the Security Council. This means they are on the Security Council forever. They were also all given equal veto power over decisions in the Security Council. These five countries were: United States, United Kingdom, China, France, and the Soviet Union.
After the Soviet Union split into a lot of smaller countries in 1991, Russia got most of the Soviet Union's nuclear weapons, and also its permanent seat in the Security Council. Some other countries also have nuclear weapons now, and can also start a nuclear war just as well, but they are not permanent members with veto power. The Republic of India and Pakistan are two countries like this.