Detinue is like when someone is holding onto something that belongs to someone else and they won't give it back. It's like if you lend your toy to your friend and they don't give it back when you ask for it. This is not okay! The friend is keeping your toy, which is called "detaining" it.
So, detinue is a special thing that happens when someone wrongfully keeps someone else's item. It's not fair, because you have a right to your own things. Sometimes people have to go to court because of this. They will try to get the item back or they might try to get money instead.
The idea of detinue is very old and comes from a time when people had to use force to get things back. Nowadays, there are courts and judges to help people who have lost something. They make a decision, and hopefully the person who has the item will give it back or pay for it.
Remember, if someone is holding onto something that belongs to you, it's important to try and talk to them first. You can explain how you feel and ask for the item back. If they won't give it back or if it's really important, then you might need to contact the police or go to court.