The film begins with Katniss taking the place of her sister, Primrose who was chosen to represent District 12 in the Hunger Games. This is an act of both sacrifice and rebellion. Katniss clarifies that she is a person of character, and she will not play the game the same as others. This continues as Katniss fails to operate in the same manner as the other participants of the Games. She isn't seeking to kill people, but to stay alive and make allies. And as she does we learn that many of the participants, specifically the ones from less wealthy Districts, aren't seeking just to kill but to live. While those called the "Careers" are all from the wealthy districts and simply want to win and will kill anyone in any way they can.
When Katniss signals to the public, who are watching live stream, a three finger symbol it causes upheaval as the hand gesture is one of great respect and love for the one who has died. So when Katniss does this for a fallen participant of the Games she is signaling unity between districts which is the opposite of what President Snow seeks. Thus, by the end of the film Katniss becomes a symbol of rebellion to all those who know that President Snow's rule is one of a tyrant that must come to an end. And Katniss will be the one to lead the people against him.
