It is also used by many other WordPress-related projects, including all plugins and themes available on WordPress.org. The GPL license gives the user of the software the rights to modify it, copy or it redistribute it, as well as any software developed based on it. Unlike many other open software licenses, GPL gives licensees the right to charge for the software. A copy of GNU GPL version 2.0 is included in every installation of WordPress. You can also read it on the official website.
According to the GPL license, any work derived from software licensed under GPL must inherit the license. That means that any code built on top of WordPress also must be licensed under the GPL. According to the WordPress license, that includes any themes or plugins built for WordPress. As noted above, that does not mean that developers are required to release their WordPress plugins and themes for free, or make them publicly available at all. It only means that licensees of those plugins and themes can build other software based on the plugins and themes, as long as the derived software is also licensed under the GPL.
The GNU General Public License was originally written by Richard Stallman of the Free Software Foundation. In addition to WordPress, it is used by the CMS Drupal, the kernel for the Linux operating system, the MySQL database management software, and many others. There are a number of licenses that are compatible with the GPL, meaning that they can also be used in projects licensed under GPL. You can see a list of the compatible licenses on the GNU projects website.