OUR MISSION
WordPress shouldn’t be expensive
Background
Most people are familiar with the term ‘open source.’ It means a piece of software, or code, that is free to use. You’re free to use, copy, study, change, improve or redistribute the software as you like.
WordPress is open source software and is founded upon the GPL (General Public Licence) that you can read more about over at the WordPress website licence page.
The main point to note is that any software that is built upon WordPress must carry forward the original GPL licence.
… we feel strongly that plugins and themes are derivative work and thus inherit the GPL license.
— wordpress.org
What this means for you
It doesn’t mean that developers can’t charge you to buy the code they have developed. What it does mean is that once you have bought that code, you are free to do whatever you like with it.
… if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
— GPLv3 licence
So, it’s perfectly legal to download the plugins and themes from this site. Download as many as you like and use them in as many projects as you want.