It is not an entire plugin but it's the only code you need in a plugin to achieve what you want. A plugin has some boilerpate code. You can see a full starter plugin at https://github.com/NodeBB/nodebb-plugin-quickstart. The plugin docs also go over the details https://docs.nodebb.org/development/quickstart/

Julian, Yeah I saw this, but in fact it's more the way it's showed that I would like I know it's just presentation but this make it easy to show which category is the most active