It's one of the fundamental texts on societal organization from a few centuries ago. It's been a few decades since I finished school, so I may misremember but IIRC:

The author believes that mankind would naturally live in a brutal state of conflict (homo homini lupus est, men are wolves to each other).

But mankind can give up their self interest and give their authority to a government/sovereign (the titular leviathan, a giant monster made of multiple people) that can rule with absolute power and guarantee an environment in which we are all better.

I cannot for the life of me imagine how this ended up on the HN front page, but it's cool.