Hey kiddo! So, do you know what a rule is? Like when your parents tell you to brush your teeth every night before bed? Well, that's a rule.
Now, imagine there are some rules that are really, really important. These rules are so important that everyone in the world has to follow them. That's called jus cogens.
Jus cogens is a fancy Latin term that means "compelling law". These are rules that are considered to be the absolute minimum standards of behavior for all countries and people, no matter what. They can't be broken or ignored, and they apply to everyone, even if they don't like them.
Some examples of jus cogens rules are things like not torturing people, not enslaving people, and not committing genocide. These are things that everyone agrees are really bad, and that no country or person should ever do.
So, jus cogens is like a super important set of rules that every country and person in the world has to follow, no matter what.