* In the book "Catching Fire," Katniss sees an Avox working in the Capitol. The Avox is a young woman, probably a former tribute who rebelled against the Capitol. She's mute, marked with a scar on her tongue, and works in silence.
* Katniss observes the Avox's plight, recognizing the cruelty of the Capitol's punishment. She also recognizes the Avox's defiance, even in silence.
* Katniss shows compassion towards the Avox, by offering her a small token of kindness, a piece of bread.
* Peeta witnesses Katniss's actions, and while he doesn't know the full story, he understands that the Avox is someone who's been wronged.
While Katniss never explicitly explains what an Avox is, her behavior and interactions with the Avox provide Peeta (and the reader) with a clear understanding of the Avox's situation, and the brutal nature of the Capitol's punishments.