Imagine you're having a sleepover with your friends and one of your friends arrives without asking your parents first. Your parents might not know that your friend is coming and they might be worried about who this person is and if they can be trusted. That's what we call an unauthorised arrival.
In grown-up terms, an unauthorised arrival usually refers to someone coming into a country without getting permission first. Countries have rules about who can come in and who can't, and if someone doesn't follow those rules, they can get in trouble.
Sometimes people might try to come to a new country because they're looking for a better life or because they're escaping danger in their own country. But it's important to follow the rules and ask for permission first so that everyone is safe and things don't get confusing or scary.