The mockingjay pin was Katniss Everdeen's tribute token in the 74th and 75th Hunger Games. An alternate version of it is featured on the cover of The Hunger Games. Unknowingly, the pin became a subtle slap in the face to the Capitol since mockingjays were created after the disastrous experiment with jabberjays and later became one of the symbols of the rebellion.
The Hunger Games
The pin originally belonged to Madge Undersee's aunt, Maysilee Donner, a tribute who died in the 50th Hunger Games. When Katniss first saw Madge Undersee wearing the pin, she thought that it could keep a family fed with bread for months due to the fact that it is made of finely crafted gold. When Madge came to bid Katniss farewell she offered it to her as a symbol of District 12 and made her promise to wear it in the arena, although in the movie, Katniss got it from Greasy Sae. Cinna later affixed it to Katniss' shirt while they were in the Launch Room, as Katniss had completely forgotten about it. He revealed that it had barely passed the review board because the Gamemakers thought the pinpoint could be used as a weapon.
Rue, who had a special relationship with the mockingjays back in District 11, decided that she could trust Katniss after seeing her wear the pin. She rebuffed Katniss' attempt to give it to her as she had her own good luck charm. Following the end of the Games, Katniss found it in her room in the Training Center just before leaving the Capitol to return to District 12.
The pin becomes a symbol of the rebellion in Catching Fire after Katniss wins the Games by defying the Capitol. However when she arrives in the Capitol on Victory Tour and later to participate in the 75th Hunger Games, she discovers that the pin has ironically spawned a new fad, being replicated as a fashion accessory and even used as the basis for tattoos, because "everyone wants to wear the winner's token." Yet this backfires on president Snow, as the mockingjay is the symbol for the rebels as well.  When the 75th Hunger Games began it was used by Haymitch to show that Finnick Odair was trustworthy.
The mockingjay pin in The Hunger Games film is based on the version which appears on the cover of The Hunger Games. The use of the pin as a tribute token is eliminated as Katniss takes an interest in the pin at the Hob, where it is among the merchandise being sold by an older woman she is dealing with and is allowed to keep it without payment. She gives it to Prim, and before she leaves to participate in the Hunger Games, Prim gives it back to her for good luck in the Games. Cinna later hides the pin within a layer of Katniss' jacket. During the Victor Crowning, President Coriolanus Snow also takes notice of the pin and examines it, something he did not do in the book.
Behind the scenes
Despite being described as a small bird in flight attached only by the tips of its wings to the ring encircling it, the mockingjay pin featured on the cover of The Hunger Games is depicted as also holding an arrow in its beak and attached to the circle by its tail. It was designed by artist Tim O'Brien, who collaborated with his wife Elizabeth Parisi, Creative Director for Hardcovers at Scholastic. She was responsible for the cover designs of The Hunger Games trilogy and has characterized the creation of the mockingjay pin as a collaboration. According to O'Brien, "It is not just a picture of a bird. It is something vulnerable. It is a symbol of the main character, Katniss, who is tough and beautiful at the same time." Before the final version was adopted, he had submitted a design for an arrow and an alternate version of the logo that had the mockingjay's right wing pierced with a chain.