Free and Open-Source (FOSS)
Free
Not necessarily free as in no-cost, but free as in freedom: free to use, study, modify, improve, redistribute, etc.
Open-Source
A project’s source code and/or design is publicly available
Why should I care?
1. Transparency
Transparency is inherent to open-source software. The source code is available for all to see, which means the author’s claims about what the software does can be validated. No funny business.
2. Security
Given the transparent nature of FOSS, there are more eyes looking for potential vulnerabilities. Yes, some of these eyes will belong to malicious actors, but a far greater number will belong to people maintaining the software; hence, it’s more likely that such vulnerabilities will be patched before they’re exploited, whereas vulnerabilities in closed-source software may only be known to the vendor (who may not patch the vulnerabilities immediately due to a false sense of security) and the malicious actors. Open-source software also resists the insertion of malicious or undesirable features, since all updates are also publicly available and verified via communal code review.
3. Privacy
FOSS fosters privacy, once again, due to its transparency. Furthermore, FOSS is often designed to be privacy-respecting first and foremost.
4. Liberty
You can do what you want with the software without any special terms and conditions. Free software won’t claim a right to whatever you create with the software.