FOSS

FOSS (or F/OSS as it is sometimes written) stands for Free / Open Source Software.

"Free" is free as in freedom, not free as in beer. Therefore free software can be expensive, but free software cannot be restrictive.

Free vs Open

One group of software developers (lead by Richard Stallman (RMS), and the FSF) care about software freedoms. They want to ensure that users of software are free to do what they want with it (provided that they don't remove the freedom of others). So for example, if RMS gives me some of his software, he wants me to be able to modify it, improve it, fix bugs, use it on whatever hardware I choose. In exchange, if I redistribute modified versions of his code, I agree to pass this freedom. This is achieved through the GNU General Public License (GPL).

Another group, prefer the word "Open", rather than "Free". They tend to be more pragmatic than "Free" advocates. There are numerous open licenses.