Adding a basic controller

This example uses ControllerBase which can greatly reduce boilerplate dependency handling code. However, it also makes the class considerably more difficult to unit test. Therefore this base class should only be used by controller classes that contain only trivial glue code. Controllers that contain sufficiently complex logic that's worth testing should not use this base class but should instead use ContainerInjectionInterface or should even better be refactored to be trivial glue code.

The content() function in the HelloController class will be returning the markup text, when the routing system invokes the page.

Within your module folder, you should have the PSR-4 standard folder structure /src/Controller and inside this folder you should have your HelloController.php controller file.
So your controller file will be like this
/src/Controller/HelloController.php

Have the following code in the HelloController.php file:

<?php

namespace Drupal\hello_world\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Defines HelloController class.
 */
class HelloController extends ControllerBase {

  /**
   * Display the markup.
   *
   * @return array
   *   Return markup array.
   */
  public function content() {
    return [
      '#type' => 'markup',
      '#markup' => $this->t('Hello, World!'),
    ];
  }

}

This code, on its own, will not do anything. It needs to be invoked by adding a routing file to our module (see previous step). Adding the controller first to our code, however, is part of a general D8 philosophy of, "Build a tool, then wire it up".