When people talk about jus in re, they are basically talking about something called "rights in things" or "rights to property." This means that you have the right to say what happens to something you own or have control over, like your toys or your room.
For example, let's say you have a favorite toy that you love to play with. You have the right to decide who else can play with it and when. That's your jus in re over that toy.
Now take that idea and apply it to bigger things, like land or a car. People can have jus in re over those kinds of things, too. That means that they have the legal right to decide what happens to them, like if they want to sell them, or give them away, or use them in a certain way.
So jus in re is really just another way of saying that people have the right to control and decide what happens to the things they own or have control over.