In the novel, the Resistance is a group of Danish men and women who band together to fight the German war machine.
The text tells us that the Resistance published De Frie Danske (The Free Danes), an illegal newspaper that reports news about Danish exploits against the Nazis. The Danish freedom fighters focus on sabotaging industrial railroad lines, factories, and German trucks and cars. The text also tells us that the Resistance is committed to bringing harm to the Nazi cause. However, it is also dangerous to be a part of the group. Anyone who is caught is summarily executed.
In the story, Annemarie knows that her parents read De Frie Danske in secret. Eventually, she discovers that Peter (her sister Lise's former fiance) is also a member of the group. Uncle Henrik explains that Peter works with him to take Jews to Gilleleje. In fact, Peter regularly brings copies of De Frie Danske to Annemarie's parents.
As the story ends, Annemarie discovers that Ilse was part of the Resistance. She died after Nazi soldiers raided the group's secret meeting place. Annemarie's mother confirms that Ilse was mowed down by Nazi soldiers in a military vehicle.
Historically, the Danish Resistance was a real movement. Many of the fighters were young, much like Peter and Ilse. They gave their lives for a worthy cause: freedom.