plugin-managerPlugins are another type of Joomla Extension. Plug-ins provide funtionality that is associated with Trigger Events. Joomla has many plugins that come with the core installation, but any Extensions that are installed may have their own plug-ins.

Some example Plug-ins would be the Search box, Authentification for Joomla, and the Captcha-Recaptcha.  Generally you don't need to enable or dissable the core Joomla plugins unless it is determined that they are conflicting with a third-party plugin. 

Occasionally, you will find that you need to enable a third-party plug-in after installing an Extension.  The Extension developer may have it installed as disabled so that it isn't using up system resources unless it it needed.

If you navigate to Plug-in Manager under the Extensions dropdown, you can click on the name of the Plug-in.  There will certain settings that you can change depending on the Plug-in itself.