Drupal, and all contributed files that are derivative works of Drupal hosted on Drupal.org, are licensed under the GNU General Public License, version 2 or later.

That means you are free to download, reuse, modify, and distribute any files hosted in Drupal.org's Git repositories under the terms of either the GPL version 2 or version 3, and to run Drupal in combination with any code with any license that is compatible with either versions 2 or 3, such as the Affero General Public License (AGPL) version 3.

The GPL requires that any recipients of a program receive its "source code". What does that mean for a web application?

• The "source code" of a file means the format that is intended for people to edit. What that means depends on the file in question.
  • For PHP code, the PHP file itself, without any compression or obfuscation, is its own source code. Note that for Drupal up to and including version 7, template files are PHP code. For Drupal 8 and later, template files are Twig code.
  • For JavaScript code, the JavaScript file itself, without any compression or obfuscation, is its own source code.
  • For CSS code, the CSS file itself, without any compression or obfuscation, is its own source code.
  • For images, the "source code" varies. Depending on the image, that could mean the production version of the file as a PNG or GIF, or an original high-resolution JPG, or a Photoshop, Illustrator, or GIMP file. The "source code" is whichever version is intended to be edited by people.