Okay, let's say you invite a friend over to your house to play. But then, you decide that you don't want them there anymore and you ask them to leave. Instead of leaving, your friend hides in your closet because they have nowhere else to go.
Now imagine your friend is a person from another country, who has come to Australia because they don't feel safe or have a good life in their own country. When they arrive in Australia, they ask for permission to stay and live here, but sometimes the government says "no".
If the government says "no", they might put your friend in a special place called immigration detention. This is like being sent to time out, but it might last for months or even years.
Immigration detention is where people who ask to live in Australia, but are not allowed to, are kept. It's usually a large building or a fenced off area. The people who are kept there are not allowed to leave, and are guarded by special people.
While they are in detention, they might have to share a bedroom with other people they don't know or have privacy. They might also have to eat food they're not used to, or might not like. They can't go outside or go to school, so they might get bored or sad.
The government keeps them in immigration detention to make sure they don't run away or cause any trouble until they can figure out what to do next. They might decide to let your friend stay in Australia after all, or they might send them back to their own country.
Overall, immigration detention is a way for the Australian government to keep people who don't have permission to live here in a safe and controlled environment until they figure out what to do next.