The concept of a plugin is to expand the functionality of Volumio.
Most of these are written by the Volumio Community, they are non official add contents and can be installed for free from the 'Plugins' section of the Volumio UI. (You need to be registered with MyVolumio account).
In the PLUGINS section you can find different plugin categories:
An example of 3rd party music plugins available for Volumio:
and many more…