When they say that Katniss Everdeen is the Mockingjay what do they mean?

Before her first Games, Katniss received a pin that had a mutt on it, a mockingjay. When she unknowingly started the rebellion by out-smarting the Capitol, the mockingjay turned into her persona. When she went into the *Spoilers* Quarter Quell Games in Catching Fire, she was rescued by the Rebellion, and they called her the Mockingjay and she was the face of the rebellion of the Districts against the Capitol and President Snow, in the book Mockingjay. :)