Rhythmbox plugins are external pieces of code that can be loaded to provide extra functionality that is not found in the normal "vanilla" Rhythmbox. Plugins can do basically everything that Rhythmbox itself could do, and in fact some "core" features like Audio CD support and Internet Radio are actually plugins (although not displayed to users as such).
Most new self-contained features, such as integration with web services and support for hardware devices, are added to Rhythmbox as plugins. Plugins can be written in C, Python, or Vala. There are sample plugins in the source tree. There's some more information for plugin developers, and API documentation.
Plugins can be distributed separately from Rhythmbox itself. If you've written an interesting new plugin, you should consider distributing it yourself, and adding a link to it on the third party plugin list.