The plugin architecture was introduced with Feng Office 1.4. As at Feng Office 2.1 the plugin framework has become more mature.
To learn what a plugin does and how it is installed the corresponding wiki page (if one available) and/or the readme file that should come with every plugin.
Once uploaded (but not necessarily installed nor activated) you can check the listing in the Plugin Manager (Feng Office 2.x) Administration → Plugins to find out more about the plugin, for example, a description, author name and website, version, etc.
Note: By default the plugin manager is not enabled. To enable it edit FENGOFFICE_ROOT/config/config.php and insert the following:
Always read the plugin documentation and follow the install instructions.
To install a V2.x+ plugin the usual method is to upload the plugin, its sub-folders and files to the plugins folder on your Feng Office Server (for example ftp://mydomain.com/fengoffice/plugins/myplugin). Then launch the plugin manager: Administration→Plugins. Find the your new plugin in the list and click 'Install' and then 'Activate'. Refresh your browser window (Ctrl+F5).
If you wnat to develop your own plugins, see the developers documentation here