In a way this is a computer programmers' in-joke, because it stands for
GNU's Not Unix
So GNU uses itself, to describe itself.
The GNU General Public Licence is the basis of 'Open Source' software.
The licence allows the user to copy, distribute and change the software freely, but they cannot sell it and they cannot sell adapted versions of the code.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: GNU