Three symbols inCatching Fireare the mockingjay, Beetee's wire, and District 13.

The mockingjay is actually a literal symbol in the novel that the characters make references to.  At first Katniss is unaware of what it means, thinking only that the Capitol has embraced it as a fashion trend, but her encounter with the escaped refugees from District 8, Bonnie and Twill who show her the mockingjay symbol baked into a cracker as a symbol of the rebellion.  Plutarch Heavensbee also shows Katniss the mockingjay symbol on his high-tech wrist watch.  Later as the novel progresses, Katniss becomes the mockingjay symbol, the symbol of the revolution, as her dress catches fire and burns away to reveal a mockingjay dress.

Beetee's wire also acts as a symbol inCatching Fire.   Beetee's plan is to connect the wire to the lightning tree to blow up the dome.  The small coil of wire represents the power of ingenuity and intellect against a harsh regime. 

District 13 is also symbolic inCatching Fire. At first, District 13 was a symbol of insurrection against the Capitol gone horribly wrong.  The Capitol used the images from District 13 as a visual reminder to the other districts of their fate if they acted against Panem.  Later, District 13 comes to symbolize the possibility of hope, as Katniss hears from Bonnie and Twill that the district was, in fact, not destroyed by the Capitol, but thriving underground.  District 13 represents the possibility and hope of a successful rebellion.