What is a Joomla! Plugin?
"A (Joomla!) plugin is a kind of Joomla! extension. Plugins provide functions which are associated with trigger events." Source: Official Joomla! Documentation.
It is not vital you know about Plugins when you first start using the Joomla! CMS.
Some knowledge of Plugins is however useful.
The Plugins Screen
To open the Plugins screen select Extensions > Plugins from the menu at the top of your website's dashboard.
Core & 3rd Party Plugins
The Joomla! CMS already contains a number of important pre-installed Core Plugins.
You will also find a long list of optional 3rd Party Plugins created by Joomla! developers in the Joomla! Extensions Directory.
Getting started with Plugins