ISIS is a big group of people who live in a place called Iraq and some other countries nearby. They have their own rules and their own way of doing things. Sometimes they use violence to get what they want, and they don't like other people who think or believe differently than they do. They also don't like the governments of some countries, and want to create their own country instead.
A long time ago, people in that area had many different religious beliefs and ways of life. Then some people started believing in Islam, and they became very powerful. They created a big empire and conquered many lands. These people were called Muslims.
ISIS is a group of people who say they are Muslims too, but they have a different way of understanding Islam than most other Muslims. They believe that their way is the only right way, and anyone who disagrees with them is wrong. They believe that anyone who doesn't follow their rules should be punished, even if it means using violence.
ISIS started in Iraq, but has spread to other countries around it. They take over towns and cities, and force people to follow their rules. They want to create their own country, where they can live under their own laws.
Many people disagree with ISIS and think they are doing bad things. Some countries have joined together to fight against them, and try to stop them from hurting innocent people. But it is a difficult problem, and many people are still affected by their violence and extremism.