What Are Plugins in Drupal 8

The Drupal 8 plugin system allows a particular module or subsystem to provide functionality in an extensible, object-oriented way

The controlling module defines the basic framework (interface) for the functionality, and other modules can create plugins (implementing the interface) with particular behaviors.

Plugins are grouped into plugin types. Each plugin type is managed by a plugin manager service, which uses a plugin discovery method to discover provided plugins of that type and instantiate them using a plugin factory.

Plugin Manager : 

Responsible for discovering and loading plugins, the most important part in any plugin type is the manager. However, it’s very simple to create one because Drupal already provides us with a sensible default base to extend.

We are dealing with the example of ice cream Shop Plugin , Code is available in my Github,
https://github.com/Dharmend/icecream  get the code and install it on d8,
when fire the url /icecream ,
it goes to Drupal\icecream\Controller\IcecreamController::page and  then
 $manager = \Drupal::service('plugin.manager.icecream');   so lets discuss the plugin manager class ,

This is required to create a new plugin type, of a plugin

class IcecreamManager extends DefaultPluginManager {

  /**
   * Constructs an IcecreamManager object.
   *
   * @param \Traversable $namespaces
   *   An object that implements \Traversable which contains the root paths
   *   keyed by the corresponding namespace to look for plugin implementations,
   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
   *   Cache backend instance to use.
   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
   *   The module handler to invoke the alter hook with.
   */
  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
    parent::__construct(
            'Plugin/Flavor', 
            $namespaces, 
            $module_handler, 
            'Drupal\icecream\FlavorInterface', 
            'Drupal\icecream\Annotation\Flavor'
            );

    $this->alterInfo('icecream_flavors_info');
    $this->setCacheBackend($cache_backend, 'icecream_flavors');
  }
}

As I mentioned, our manager extends the DefaultPluginManager class and just overrides the constructor to call the parent one with some important information about our plugin type:

'Plugin/Flavor' – the subdirectory where plugins of this type will be found within any module,
Drupal\icecream\FlavorInterface – the interface each of our plugins will need to implement,

Drupal\icecream\Annotation\Flavor – the annotation class that will define our plugin properties (such as ID, name, etc.)

Additionally, we create an alter hook which can be implemented by various modules to alter the plugin definitions and we set a key for our plugins in the cache backend. 

Reference : https://www.sitepoint.com/drupal-8-custom-plugin-types/