Contributing To WordPress: A Beginner’s Guide For Non-Coders
If you’ve been using WordPress for any amount of time, there’s a good chance you’ve come across the following statement: “Free as in speech, not free as in beer.’ If you haven’t, pull up a chair and let’s talk.
WordPress is a free and open-source software (also known as FOSS) project. The explanation of that could easily fill up a separate article, but the TL;DR version is that the software is free to download, use, inspect and modify by anyone who has a copy of it.
Source: Smashing Magazine