GPL stands for General Public License and is sometimes written GNU GPL because the GPL was developed by the GNU Project. The first GNU license was created by Richard Stallman of the GNU Project in in 1989. The second version (GPLv2) was released in 1991 and the third version (GPLv3) was released in 2007.
A GNU license is what is used by WordPress and other types of free software. The term free software means much more than just the ability to download and use the software without having to pay money. The term free software also means that you can access the core code and have the right to make modifications or derivative products.
When you install WordPress, a copy of the GPL license will be included. Current distributions of WordPress are licensed under the GPLv2 (GPL version 2) of this license which places certain restrictions on how the software can be modified and distributed. Specifically, anything designed for use with WordPress like a theme or plugin must be distributed under the same GPL license.
If you’re not planning on developing your own themes or plugins, or if you do plan on creating derivative products but aren’t going to distribute or sell them to other people, then you don’t need to worry about the GPL license. If all you are doing is operating a WordPress site, then the GPL license is largely irrelevant.
But for developers who plan to sell their works or distribute their works for free, it is essential that every aspect of your product be GPL compliant. This means that any CSS files, images, or other parts of your product are also free software and legally able to be distributed with the GPL license.