What does #markup mean?

I meet this array key all the time in Drupal (for example in Symphony's dpm()) yet I am not able to find any documentation about it, what it does, how can I use it? How do I display something that is given as #markup value in Twig?

By using #markup in render arrays you can output raw HTML (passed through XSS filter to ensure security).

Here's an advanced example, with optional #allowed_tags and placeholders.

/**
 * Implements hook_preprocess_HOOK().
 */
function mymodule_preprocess_foobar(&$variables) {
  $variables['video'] = [
    '#markup' => '<span>@video_label:</span> <video><source src="v.webm" type="video/webm"></video>',
    '#allowed_tags' => [
      'video',
      'source',
    ],
    '#attached' => [
      'placeholders' => [
        '@video_label' => t('Sample label'),
      ],
    ],
  ];
}

In Twig you just do:

{{ video }}

And in your browser you get the HTML with replaced placeholder and stripped <span>:

Translated label: <video><source src="v.webm" type="video/webm"></video>

Just by using the regular print twig syntax: {{ variable }}

The #markup is the html that will be on the final page sent from the server.

You can use a preprocess function to intercept and change the #markup if you need to.