It refers to the established theme of the mockingjay bird that was developed in the other books of the trilogy. In the Hunger Games arena, Katniss became identified with the mockingjay and it turned into a kind of personal symbol for her.
Katniss becomes a symbolic mockingjay for the rebels in their fight against the Capitol. Mockingjay becomes her title too; Katniss is referred to as the Mockingjay just as often as she’s called her real name, and is presented as the Mockingjay to people across Panem.
As well, Katniss embodies this Mockingjay in more ways than one. Just as the real mockingjays can only sing the words of other people’s songs, Katniss is forced to speak the words and ideas of the revolution. As the Mockingjay, she’s still being manipulated, just by District 13 this time instead of the Capitol.