Operation Payback was a protest on the internet that happened a long time ago. The protest was about something called "Internet Freedom." People who did the protest wanted the internet to stay free, and they didn't want anyone to tell them what they can or can't do on the internet.
Some people who were part of the protest wanted to help out a person who was in jail. They thought that the person was put in jail unfairly because he did something on the internet that the government didn't like. Other people in the protest were angry about big companies that were controlling the internet and telling people what they can do on it.
To do the protest, people on the internet got together and did something called "Distributed Denial of Service" or DDoS. This is a big word that means they kept sending lots and lots of requests to websites that they didn't like. It was like having so many people try to open a door at the same time that the door couldn't open anymore. That's why the websites that they were protesting against couldn't work.
The protest started small, but it got bigger and bigger until a lot of people were involved. There were even some big companies that joined in on the protest. However, the police didn't like the protest, and they tried to stop it. They arrested some of the people who were part of the protest, and some of the group's websites got taken down.
In the end, the protest stopped, but it showed that people can come together on the internet to fight for things that they believe in.