Okay, so you know how some people really care about the environment and want to protect it? Some of these people might feel like the only way to get attention and make change is by doing extreme things that are illegal and dangerous. That is what is called ecoterrorism.
In fiction, this means that there are stories where characters who care a lot about the environment are doing things like breaking into factories or chemical plants, or sabotaging machines that are polluting the air or water. Sometimes they might even hurt people, which is not okay in real life, but can be part of a story.
Now, it's important to remember that in real life, doing illegal things isn't the right way to make change. We should always try to make change peacefully and within the law. But in some books or movies, ecoterrorism is used as a way to show how strongly someone cares about protecting the environment, and the consequences that can come from trying to do things illegally.