what is Theme Hook Suggestions?

This hook allows any module or theme to provide

• alternative theme function or template name suggestions and
• reorder or remove suggestions

provided by hook_theme_suggestions_HOOK()

//Drupal 7:

  <?php
  /**
  * Implements hook_preprocess_HOOK() for node templates.
  */
  function MYTHEME_preprocess_node(&$variables) {
    $variables['theme_hook_suggestions'][] = 'node__' . 'first';
    $variables['theme_hook_suggestions'][] = 'node__' . 'second';
  }
 

//Drupal 8:

  <?php
  /**
  * Implements hook_theme_suggestions_HOOK_alter() for node templates.
  */
  function MYTHEME_theme_suggestions_node_alter(array &$suggestions, array $variables) {
    $suggestions[] = 'node__' . 'first';
    $suggestions[] = 'node__' . 'second';
  }
  ?>
 
hook_preprocess for suggestions
Each theme hook suggestion will automatically get it's own preprocess function. For the example above, the following two preprocess functions will be available:

/* for $suggestions[] = 'node__' . 'first'; */
function hook_preprocess_node__first(&$variables);

/* for $suggestions[] = 'node__' . 'second'; */
function hook_preprocess_node__second(&$variables);