The mockingjay pin is the circular gold token Katniss wears during the Games to represent District 12. Each tribute is allowed one item to remind them of their friends and family in their home district; the small pin bearing the image of a bird in flight is Katniss's.
The mockingjay pin was originally given to Katniss by Madge, the mayor's daughter, before Katniss's departure to the Capitol to participate in the Hunger Games (3.24). Though Katniss forgets about the pin during her training sessions, Cinna, her stylist, is sure to place the mockingjay prominently on Katniss's outfit for the arena. He tells her that the little pin "barely cleared the review board" since the authorities thought Katniss might be able to use the pin as a weapon (10.94). While the thought of a tiny little pin being used as a weapon might seem silly to us, the image of the mockingjay, as we will find out, is indeed an incredibly powerful and important weapon.
What is a mockingjay, you ask? Well, as we are informed in Chapter 3, a mockingjay is a bird, of course, but a hybrid one. Their lineage can be traced to a species known as the jabberjay. Jabberjays were birds that had been genetically engineered by the Capitol as spies during the rebellion of the 13 districts. The little guys were meant to overhear subversive conversations, and bring that information to the Capitol, and repeat what they heard. This was all fine and dandy, of course, until the rebels caught on and started feeding the jabberjays false info. Once the Capitol figured this all out, the birds became useless to them. The jabberjays were left alone in the wild where they mated with female mockingbirds. After a few generations, the mutated birds could no longer repeat or carry information, but they instead learned to carry a tune and to repeat songs and melodies. Hence, the mockingjay was born.
The mockingjays appear not only on Katniss's pin, but in several portions of the novel. We learn, for example, that the bird reminds Katniss of her father, who loved mockingjays very much, and loved to whistle and sing with them (3.57).
Rue, another music lover, is also associated with mockingjays, since she sings to the birds in order to send messages to the other agricultural workers back in District 11 (16.33). After Katniss and Rue team up in the arena, Rue teaches Katniss one of her mockingjay signals, which, if Katniss hears it, should mean that Rue is alive and well. Unfortunately, that same signal will later lure Katniss into a trap, where Rue is being held hostage by the tribute from District 1. Katniss will arrive too late to save Rue's life. As Katniss sings a funeral song for Rue, the mockingjays will take up the song and spread it through the forest (18.32).
Katniss's mockingjay pin, like the bird itself, symbolizes a creature with a spirit of its own. As hybrid creatures that have broken free of the control of the Capitol, they suggest the inability of the Capitol to enforce their power over all living creatures. These birds are signs of resistance and rebellion. As Katniss says, they're "something of a slap in the face to the Capitol" (3.55). In many ways, as we will see in the novel, Katniss herself is very much like a mockingjay and will come to identify (or be identified with) the bird.