The priority of justice
Rawls's loudest thematic thesis is that justice needs to be elevated in private and in public spheres to a position of priority and ultimate importance. He feels humans should stand up for justice with passion and zeal, and he also feels justice is an organism, so to speak, which can heal itself and adjust for its mistakes, but only when the public discourse around justice is encouraged, and when people are enfranchised to power.
The nature of human connection
Humans are intimately connected in biological, psychological ways. For Rawls, that is the essential nature of justice, to level the playing field between people, first by prioritizing justice above personal gain, and also by understanding the people in one's own life. In Rawls opinion, injustice happens everywhere all the time, because people don't understand the way they use one another for their own interests, prioritizing themselves over others.
Curating a healthy society
This book is an attempt to do this very thing, which it also advocates to the reader. Instead of looking at society as a solid thing that exists without our control, humans should understand that they are very powerful, and by standing up for justice in their own lives, they can set off consequences that help their society to be more fair in general. When one person sees another person standing up for the right thing, they become more likely to stand up for their beliefs too.
