This folder holds the plugin files. Plugins allow to add third-party extensions to the application.
Plugins can be a single file with the extension `.plugin.php` or a sub-folder with the suffix `_plugin`. It is recommended to use a sub-folder for each new plugin you may develop. You may also place each old single-file plugin into a folder.
You can add new plugins by copying it into this folder. Once the files are in this folder you still need to install the desired plugins through the back-office in order to activate them.
Depending on the activated plugins, this folder may or may not need to be accessed through the web.
A global overview of the codebase can be found here.