Template files are responsible for the HTML markup of every page generated by Drupal. Any file ending with the .html.twig extension is a template file.

These files are composed of standard HTML markup used by the Twig template engine

In this article, we need understand:

• What template files are, and how they fit into the big picture of creating a theme
• How template files are used in order to allow theme developers to modify the HTML markup output by Drupal
• Naming conventions for, and specificity of, template files