Mockingjays are the result of the genetically created Capitol jabberjays mating with female mockingbirds and creating a unique species. After the emergence of mockingjays, their jabberjay progenitors became "as rare and tough as rocks," as Katniss stated in Catching Fire. Mockingjays lost the jabberjay's ability to enunciate words, but can copy perfectly, down to the last note, any human tune. If a singer with a voice the mockingjays respect sings, they will fall silent. Katniss, Peeta, and Peeta's father note this throughout the series. Katniss, her father, and Rue are singers that have caused mockingjays to fall silent as mentioned in Mockingjay and The Hunger Games. District 11 is known to have an especially large mockingjay population, as confirmed by District 11 tribute Rue who has special mockingjay friends.
Mockingjays acquire a symbolism throughout Panem following the 74th Hunger Games. In the Capitol the pin stands for a very exciting Hunger Games. For the rebels it is a symbol of rebellion. At the beginning of The Hunger Games, Katniss was given a mockingjay pin by Madge Undersee, the daughter of District 12's mayor. She recognized the bird not immediately but while she was waiting for guests to say their final goodbyes before the opening of the Games, and said that it was a huge "slap in the face" to the Capitol, because mockingjays were never intended to exist. Katniss wears the pin as her token in the Games, and by Catching Fire it becomes a symbol of rebellion. In Mockingjay, Katniss becomes the titular character, a person who speaks to the districts for the rebels, and she wears a mockingjay-inspired costume and the pin. - Wikipedia
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