Get information about a menu item In Drupal 8, you can use system blocks to show menus starting from different levels. For example:

Menu item 1 (level1)
--- Menu Item 2 (level2)
--- Menu Item 3 (level2)

Menu item 4 (level1)
--- Menu Item 5 (level2)
--- Menu Item 6 (level2)


When user is on a page linked by menu item 2 I want to have a block showing the following:

Menu item 1 
    --- Menu Item 2
    --- Menu Item 3


What the main system block offers is the following result:

    --- Menu Item 2
    --- Menu Item 3


You see? I need the parent menu item of these menu items as well.

What I did so far is to define a custom block in my module:

class MainMenuBlockSecondLevel extends BlockBase {

  public function build() {
    $menu_name = 'main';
    $menu_tree = \Drupal::menuTree();
    $parameters = $menu_tree-&gt;getCurrentRouteMenuTreeParameters($menu_name);
    $parameters-&gt;setMinDepth(2);

    $tree = $menu_tree-&gt;load($menu_name, $parameters);
    $manipulators = array(
      array('callable' =&gt; 'menu.default_tree_manipulators:checkAccess'),
      array('callable' =&gt; 'menu.default_tree_manipulators:generateIndexAndSort'),
    );
    $tree = $menu_tree-&gt;transform($tree, $manipulators);

    $parents = $parameters-&gt;expandedParents;  // menu_link_content:dd7b39a6-c4c8-48dd-a1cf-92a755087c29


    $build['parent']['#markup'] = '&lt;div class="parent-menuitem"&gt;Parent&lt;/div&gt;';
    $build['menu'] = $menu_tree-&gt;build($tree);
    return $build;
  }
}


With the $parents variable I get an array like that:

Array
(
    [0] =&gt; 
    [1] =&gt; menu_link_content:dd7b39a6-c4c8-48dd-a1cf-92a755087c29
)


Here it stops for me now. What is menu_link_content:dd7b39a6-c4c8-48dd-a1cf-92a755087c29 and how can I get the needed information like menu item title and path out of it?
MenuTreeParameters::$expandedParents are a list of menu tree ids (a composite key of the menu bundle and the menu content UUID). You can see its data in the menu_tree, menu_link_content, menu_link_content_data, database tables. 

If you need to programmatically invoke the menu item for reference, you can load it via Drupal's entity manager:

// Load each menu content item per UUID
foreach ($parents as $menu_uuid) {
  $menu_content = current(\Drupal::entityManager()-&gt;getStorage('menu_link_content')-&gt;loadByProperties(array('uuid' =&gt; $menu_uuid)));
  // @TODO: Reference menu content properties for data
  // (e.g. $menu_content-&gt;get('title')-&gt;value / $menu_content-&gt;get('link')
}