Katniss took a mockingjay pin with her into the arena with her during her Hunger Games. (Psst. You can read all about Katniss' pin here.) Since then, Katniss has been associated with the mockingjay. Now, however, it's taken on a life of its own:
My bird has been replicated on belt buckles, embroidered into silk lapels, even tattooed in intimate places. Everyone wants to wear the winner's token. I can only imagine how nuts that makes President Snow. But what can he do? The Games were such a hit here, where the berries were only a symbol of a desperate girl trying to save her lover. (6.15)
What else could Katniss' mockingjay be a symbol of? Rebellion against the Capitol, of course. That's why, in the oppressed districts of Panem, the mockingjay is taking on deeper significance.
The mockingjay starts to pop up everywhere. It's on the cracker that Bonnie and Twill hold out to prove their anti-Capitol alliances. It's the sign that revolutionaries like Plutarch use to signify their membership in what's rapidly becoming a popular, not elitist, club. It soon comes to signify the revolution itself across the districts, even though the people in the Capitol may not recognize it as such:
A shadow of recognition flickers across Caesar's face, and I can tell he knows that the mockingjay isn't just my token. That it's come to symbolize so much more. That what will be seen as a flashy costume change in the Capitol is resonating in an entirely different way throughout the districts. (18.3)
When Katniss' wedding dress changes into a mockingjay costume, it also reinforces the idea that she is the mockingjay, a sentiment Plutarch confirms at the end of the book: "We had to save you because you're the mockingjay, Katniss [...] While you live, the revolution lives" (27.41). The mockingjay, Katniss, and the revolution have all become one and the same. It's a connection of images that will only grow stronger in the trilogy's final book, Mockingjay. (For more on how Katniss acts as a mockingjay, check out "Characters: Katniss Everdeen.")