A Girl on Fire?
Here's an example of one of the eye-catching outfits Cinna designs for Katniss. This one in particular leads to her being called the Girl On Fire. As to exactly why that is, you'll have to see the movie...
Another important symbol from the movie and book is a pin given to Katniss; a Mockinjay pin - a bird encased in a golden circle. The story as to how she obtains the pin is different in the movie, but it becomes a central theme as we learn that the Mockingjay is a genetically enhanced bird, a combination of a Jabberjay and Mockingbird, created by the Capitol many years ago.
It's an anti-Capitol symbol and Katniss wears hers with definance to prove that they can never have complete control over her.