In “Mockingjay — Part 1,” Katniss is followed by a film crew who are making pro-rebellion propaganda. They film her singing the song, and it’s quickly broadcast throughout the country and used as a rallying cry by rebels. But the original books give us more insight into the song’s history and role in the narrative.
In the books, Katniss remembers that her dad (who has died by the time the books begin) would often sing “The Hanging Tree” and taught it to her and her sister, Prim (Willow Shields). When her mom found out, she was furious and banned the song. Peeta also remembers Katniss’s dad singing the song, and it’s a positive memory for him of their past.
Katniss ultimately reconsiders the song and realizes the narrator is just trying to protect his love. The song also appears to be banned in all of Panem because the real “lover” is not one person, but all would-be rebels, who ought to fight for freedom, even if it means death.
The book “The Ballad of Songbirds and Snakes” reveals that Lucy wrote “The Hanging Tree.” Spoilers: Lucy bases the lyrics on a real man in District 12 who’s executed after being blamed for the deaths of three men. The song also alludes to the romantic connection between Lucy and Coriolanus Snow (Tom Blyth in the new film and Donald Sutherland in the original four). They agree to meet at the hanging tree to run away together.
Lucy is a member of a group called the Covey, nomads known for their musical talent. After the events of “The Balled of Songbirds and Snakes,” the Covey are banned from performing ever again. Katniss’s dad then only could have learned the song through a network of ancestors who passed it through the generations in secret.