Okay kiddo, so there are some rules that people have to follow when they come to live in a new country. One of those rules is called mandatory detention. When someone arrives in a new country, the government might make them stay in a special place called a detention center for a little while. This is because the government wants to make sure that the person is not going to cause any trouble or harm to other people in the new country.
Sometimes people have to go to mandatory detention if they don't have the right paperwork or if they are waiting for permission to stay in the new country permanently. It's kind of like when you get sent to time-out when you do something wrong. You have to stay there for a little bit until you learn how to behave better.
Now, some people don't think that mandatory detention is fair because it can make people feel trapped or scared. And sometimes people have to stay in detention centers for a really long time, which can be hard for them and their families. But other people think that it's important to make sure that everyone who wants to come to the new country follows the rules and doesn't cause any harm.
So, that's what mandatory detention is all about, little one. It might seem a bit confusing or scary, but it's just a way to help keep everyone safe and make sure that people follow the rules when they come to live in a new country.