A WordPress plug-in is an add on to the core files of WordPress in order to expand its features. There are free plugins & there are premium plugins which need to be purchased from the developer.
Plugins can be found in one of two ways, the first by visiting the WordPress main website. The second, if you have a WordPress installation yourself, is through the admin panel of your site. Under the plugins section you can see all the plugins you have installed and you can add new plugins by searching the main WordPress website.
Installation of new plugins are easy. Simply search the WordPress plugins database, locate the plugin you require and click on install. The plugin is automatically installed. You will then need to activate the plugin. Some plugins need to be configured and you will find those in your admin panel.