Why do people argue? It is because people don't agree all the time. Sometimes people argue and then get over it quickly. Other times people argue and won't give up. They just keep arguing.
Sometimes arguments can turn into something bad. People can get very angry when they are arguing. People may shout. They may fight. They can even get hurt. Those kinds of arguments are bad.
Sometimes entire countries argue. Two countries or a lot of countries will disagree. Maybe they disagree about a land border. Maybe they disagree about how to live. Often other countries take sides. Arguments between countries can turn into fighting, too.