sovereignty
It is a fact of civilized life that there exist rules which govern and constrain individual behavior. You can't steal your neighbor's car because you don't have one, or because his is faster. Regardless of one's moral philosophical framework, just about everyone agrees on that. We tend to put the people who don't in big buildings with thick walls and high fences so that they can't get out.
A slightly more interesting set of questions is how rules govern collective behavior. How do these rules come about, what are the best rules, and who gets to decide what they ought to be? We still haven't got the right answer; at least, it wouldn't seem so, given that there is no global agreement on questions like whether it's okay for one nation-state to invade another nation-state and take their stuff.
If a collection of people agree on a set of rules, and one of them breaks one of the rules, there are usually agreed-upon consequences. But if a collection of collections haven't yet formalized the rules, or they did a shoddy job, and a sub-collection does something that the broader collection thinks is not okay even though the sub-collection disagrees, what right does the broader collection have to impose consequences on the sub-collection if they never agreed on what's okay and not okay in the first place? It gets tricky.
Sovereignty is the idea that an individual, or a collection, or a collection of collections, gets to decide for itself what the rules are, and no higher authority has the right to intervene. Here in the 21st century, we're still trying to balance the interest of sovereignty with the interest of global order. Are we getting the balance right?