The movie is named from the nicknames of the two main characters. Flame refers to the colour of Bent Faurschou-Hviid's red hair, after an attempt to dye it blond. Citron (Danish for lemon) got the name because, while working at the Citroën-factories in Copenhagen, he sabotaged German cars and trucks.
The trivia items below may give away important plot points.
The scene where the German soldiers mock the corpses of Flame and Citron is based on how when the Germans took Flame's body out of the house, they reportedly cheered at the sight of the corpse. The soldiers then dragged Flame downstairs by his feet, bumping his head into the stairs continuously.
When Flame is trapped in his home, he goes directly to the basement where he commits suicide. In reality, he tried to escape by going upstairs before realizing he was surrounded. He then took a cyanide capsule while upstairs.