Some websites and software products take pride in using only free software. This means the user is welcome to see the code, understand how it works, and, if desired, modify the code.
For a good resource about the GPL, check out this article from Drupal.
If you use WordPress, or another popular content management system, you can view the source code on the project website. Without having to interpret the code yourself, you can view the software license. For example, WordPress is licensed under the GNU General Public License (GPL). This means the source code should be safe enough for you to ensure your users of software freedom. The code is available for independent review, without obfuscation, and therefore hides nothing from its users.
However, if you add additional themes or plugins to your site, as is likely, you will want to check those software license agreements as well. Derivative works associated with free software, depending on the license, are not required to use the same license or be bound by the same restrictions.