FLOSS stands for Free 'Libre' Open Source Software. It is a term that is used to represent software that has very limited restrictions on its license. Users of FLOSS are given the right to read and alter its source code. It is important to remember that free software does not mean that the software is free of charge. It means that the software is free from restrictions. Think of it in terms of "free speech" rather than "free food". Of course, there's lots of software out there that can be both. Just imagine a world where you don't have to simply file a bug report on the software that you use and then wait for the manufacturers to get back to you (which could be a very long wait!). Perhaps, if you have the technical knowledge, you could add to the software or call upon others to try and improve it. This would benefit not just you but all other users of that piece of software. Folks used to call such software that is free of restrictions as FOSS without the 'L'. However, this caused some level of ambiguity since it led to the assumption that one did not have to pay for the software. Such a notion would have spelt doom for any business prospects that such software might have. Once the word 'Libre' (meaning Liberty in Spanish or French) was brought in, the meaning became clear. So what FLOSS can you get? Well, apart from your dentist, here's an extensive list compiled by Wikipedia. I'm not too much of an activist. So, I wouldn't tell you to get rid of your proprietary (possibly pirated?) software. You probably use free software anyway. If you use Firefox, VLC or Gimp, then you are already a part of the solution. There are other useful applications that you could use. All it takes is an open mind to make a difference.
Other important links:-
Free Software Foundation
Info on Free and Non-free Software Licenses
Making the Move