Software that has a source code available to the public.
Giving the public the ability to modify, improve, and fix the program itself. Allowing the creator of the program to not have to find a way to fix the program himself and allowing more patches and improvements to the software.
Also allowing the public to be able to port the program onto other operating systems, so that dual-booters who can program will make a windows version of an awesome GNU/Linux/BSD/Solaris program.
See also open source
My Slackware has the linux source code locate in "/usr/src/linux". I compile my own linux kernal to fit my needs of my computer. Take that M$!
GPL is an example of an open-source license.
Prices shown in USD.