It's a history of macro trends in violence - wars, homicide, rape, etc. The burden of violence in the modern world is much, much lower than it was historically, even fairly recently.
The book catches a lot of flak from people who reject the claim that the future will automatically be more peaceful than the past. I think this is poor criticism because that's not what the book says; it is explicitly a descriptive history and not predictive.
I love this book because it presents remarkable evidence from multiple fields that the world has gotten profoundly better (at least, regarding violence). The realization that the world can improve and has improved is...liberating? Surprisingly many people don't believe this, though I expect on HN belief in progress is not uncommon.
Having such evidence that the world has improved so much is powerful motivation to try to continue - you know it's possible. It's the antidote to incorrect zero-sum thinking, which is not just damaging but also wrong. And perhaps it's a moral call to action: our grandparents' and parents' generations left us a world which is much more peaceful than the one they were born into. Do we not owe future generations the same gift?