Unlawful combatants are people who fight in a war but do not follow the rules that are agreed upon by countries that are fighting. It's like playing a game with your friends, but you don't follow the rules that everyone agreed on. This means that they may do things like attack innocent people or use weapons that are not allowed.
This is a problem because when countries agree to go to war, they also agree to follow certain rules to make it fair and keep people safe. Unlawful combatants are people who break those rules and put innocent people in danger.
If someone is captured and found to be an unlawful combatant, it means that they may not be treated the same way as other prisoners of war who did follow the rules. They could be punished more severely because they broke the agreed-upon rules of war.