https://www.drupalatyourfingertips.com/render

Overview

Render Arrays are the building blocks of a Drupal page. A render array is an associative array which conforms to the standards and data structures used in Drupal's Render API. The Render API is also integrated with the Theme API.

<?php

namespace Drupal\hello_world\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\hello_world\HelloWorldSalutation;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
* Controller for the salutation message.
*/
class HelloWorldController extends ControllerBase {

 /**
  * The salutation service.
  *
  * @var \Drupal\hello_world\HelloWorldSalutation
  */
 protected $salutation;