So the page you're looking at shows how to convert a tpl.php to a html.twig. This is showing you how to update a Drupal 7 theme to be compatible with Drupal 8.
Assuming that is in fact what you're trying to do, you will normally find Drupal 7 theme templates at sites/all/themes/YOUR_THEME_NAME/templates. Once you navigate there, you should see files ending in tpl.php. Inside these files you might find a theme function being used. You can identify them as they'll look something like function theme_YOUR_THEME_NAME_hook($variables).
What is a theme function?
A theme function is a PHP function that exists to provide HTML that is can be customized by themes and modules.
Here is the official list of theme functions. You can find a lot more information on Drupal.org, but here are a few starter links:
1. Theme API
2. Using the theme layer
3. Theme hooks
I would highly recommend reading a book on theming before tackling anything else. Drupal has a lot of depth, and if you're truly on day one and already converting themes, this will serve you well. I recommend 'Using Drupal' to understand the basics of Drupal, as a lot of Drupal development can be accomplished through the User Interface. Then move onto 'Drupal 7 Development by Example Beginner's Guide' to gain an understanding of how and when you should delve into actual code development.
