Okay, so imagine there were two groups of people called the Nazis and the Fascists. They were in charge of Germany and Italy a long time ago.
Sometimes, countries work together and help each other out. But in this case, some other countries decided to work with the Nazis and Fascists. This means they agreed to do things together and help each other, which is called collaboration.
Collaboration with Nazi Germany and Fascist Italy means that other countries kind of became friends with them. They agreed to support and assist them in different ways.
Some countries collaborated with the Nazis and Fascists because they thought that by being friends, it could benefit them in some way. They might have thought they could get something they wanted, like more power or land.
People in those collaborating countries might have believed the ideas and beliefs of the Nazis and Fascists, or they might have been forced to go along with them. This is not a good thing because the Nazis and Fascists did some really bad things to people they didn't like, like hurting them and even killing them, just because they were different.
Collaboration with Nazi Germany and Fascist Italy is seen as a very bad thing because those countries were doing terrible things. It's important to remember that it's always better to stand up against injustice and help the people who are being treated unfairly.