A complaint system is a way for people to tell someone in charge when they are unhappy with something. It is a way to make sure that things are fair, that people are treated fairly and that people can get help if they need it.
Let's say you go to a store and someone is treating you badly. You can use the complaint system to let the store's manager know about it. The manager can then investigate and see if the person was really being mean and can take action if needed. That way, people know that if something is wrong, they can tell someone and the problem can be fixed.