This hook fires for pretty much everything WordPress does, every time anythign is displayed there is a search of some sort going on. You can use it to alter the results the search returns.

The $Query object class

The $Query object that is passed to the hook shouldn’t be altered, but can be used to do lots of things – see here.

Examples

Filtering search results to user doing a search using a search box

add_action('pre_get_posts', 'my_pre_get_posts');
function my_pre_get_posts($Query)
{

  //Do not change queries in the admin screens
  if (is_admin())
    return;
  
  //Only modify the main search query on the front end
  if (!$Query->is_main_query())
    return;

  if ($Query->is_search())
  {
    //----------------------------------------
    //----- THIS IS A SEARCH BY THE USER -----
    //----------------------------------------

    $UrlWithoutArguments = parse_url($_SERVER["REQUEST_URI"], PHP_URL_PATH);    //Get page without any url arguments

    if($UrlWithoutArguments == '/my_search_results')
    {
      //USER DOING A SEARCH ON PAGE /my_search_results

      //Filter by author user_id's
      $AllowedUserIds = [2, 5, 10]; // Replace with your desired user IDs
      $Query->set('author__in', $AllowedUserIds);
      
    }

  } //if ($Query->is_search())
}