Okay kiddo, so Tarnac Nine is a group of people who live in a place called Tarnac, which is in France. They are called Tarnac Nine because there were nine people who were accused of doing some bad things, like hurting the train tracks and causing problems.
The government thought that the Tarnac Nine were part of a group that wanted to cause chaos and do things that were not good for the country. But the Tarnac Nine said that they were just friends living in a small town and they did not do anything wrong.
There was a big trial to decide whether the Tarnac Nine were guilty or not. Some people thought that they should be punished, while others thought they were innocent. In the end, it was decided that they were not guilty and they were allowed to go home.
So, that's basically what Tarnac Nine is. A group of people who were accused of doing bad things, but were later found innocent.