Katniss survived two Hunger Games. Her district, however, did not: it was bombed shortly after her entering in the arena. The few survivors of District 12 took refuge in District 13, which was thought to be deserted but is in fact hosting the rebels. And this is essentially what this third volume is about: the rebellion against the Capitol, with Katniss as their Mockingjay, human symbol of the resistance who would apparently prefer to be "useful" in the battle than "useless" in front of a TV camera.
I was not convinced by this conclusion to the series. I found the book to be messy and confusing, and that the characters lacked... I don't know, depth? They all kind of feel all clones of each other, which may be the intention, but in the end made things both confusing and bland (because I didn't really care for them anymore).
The story in itself is satisfying enough - it finishes properly, it gives some closure, but it definitely felt rushed.