Okay, so you know how when you play a game, there are rules you have to follow? Just like that, there are rules that countries follow when they want to say how things should be done internationally. These rules are called "conventions" and one of them is called the Leiden Convention.
The Leiden Convention is a special set of rules that helps different countries respect each other's legal systems. So, imagine if you went to a different country and you broke a law there - even if you didn't know it was wrong! The Leiden Convention helps make sure that you get treated fairly and that your own country's legal system can help you if you need it.
It also helps protect people who are involved in legal cases - like lawyers or witnesses - and keeps everyone's personal information safe.
Overall, the Leiden Convention is like a big agreement among countries that they will all play fair and be nice to each other when it comes to legal stuff.