What plugins are
Extra features you switch on, and what happens when you do.
A plugin is a self-contained extra: new kinds of content, new screens, new settings. Under Configure, Plugins lists everything installed with an Enable or Disable button.
Switching one on takes effect immediately — nothing restarts and your site does not go down. Its screens appear in the sidebar under Plugins.
Disabling is safe
Disabling a plugin hides its screens and stops its features. It does not delete anything it created — your content stays where it is, and switching the plugin back on brings the screens back with everything intact.