The Charter of the United Nations is like the rules of a big club that almost every country in the world is a part of. Just like how you have rules in your class, like no shouting or hitting, the countries with the United Nations also have rules to follow.
When the big war ended a long time ago, the leaders of many countries got together to make sure that such a terrible thing never happened again. They agreed on a bunch of rules to make sure countries were nice to each other and didn't fight all the time.
The United Nations works like a big family, with all the countries sitting together and talking about things that are happening around the world. They are trying to solve problems, like how to help poor people have better lives, or how to stop wars from happening.
Everyone in the United Nations has to agree on what to do, and nobody can just do what they want. If somebody breaks the rules, the other countries will work together to stop them and make them behave better.
So basically, the Charter of the United Nations is a set of rules that countries follow so that they don't fight with each other and can work together to make the world a better place.