Well, kiddo, during World War II, resistance was a group of people who didn't like the bad guys and wanted to stop them. These bad guys were called Nazis and they were trying to take over other countries and do very terrible things to people they didn't like.
The resistance was made up of regular people who joined together to fight against the Nazis. They did sneaky things like blowing up bridges and factories that the bad guys used, stealing their weapons and supplies, and spreading messages to help the other good guys know what was going on.
It was like a secret club where everyone had to be quiet and only talk to others they trusted. They were very brave and put themselves in danger to help make the world a better place. Some of them even sacrificed their own lives to protect others.
The resistance was a very important part of the war because they made it harder for the bad guys to do their bad things. They helped other people have hope and gave them courage to keep fighting too. And in the end, with the help of many brave people, the good guys won and the world became a better place.