Jus legationis means the right to send representatives to another country.
Okay, so let's say you have a friend who lives far away. You really want to talk to them and find out what's going on in their life, but you can't just go visit them whenever you want. That's because you need to follow certain rules and have permission to go to another country.
Similarly, countries have rules about who can go visit them and talk about different things. Sometimes, when countries want to talk to each other about important stuff like trade or peace, they will send representatives to talk. These representatives are called ambassadors.
Now, to send an ambassador to another country, you need permission from that country. This permission is called jus legationis. It means that the country agrees to let your ambassador come visit and talk to their people.
So, jus legationis is like asking permission to send your message to another country through a special person called an ambassador. It's like making sure you follow the rules and play nicely with other countries.