How does Free and Open Source Software differ from Closed Source software? Please explain to me why (or if) it is important for society.
user
Free and Open Source (FOSS) software is released under the premise that it is truly free, in the sense that it is both free-of-charge to use and that the source code is freely available to view, modify, and compile. This means that the likelihood of any hidden malicious or unwanted behaviour is drastically reduced, since it is trivially easy to detect and remove such unwanted behaviour. Open-source projects are often maintained by groups of volunteers in their spare time, which means that the project is unlikely to be discontinued, as long as there is interest in it. On top of this, open-source software is often "forked", or cloned into other software, which allows for improvements to be made separately or for the software to be used as a template for other software. For example, a lot of popular web browsers, such as Opera, Vivaldi, Microsoft Edge, and Google Chrome are forks of the Chromium web browser, which is open-source. Firefox also has several clones, such as Kiwi Browser and LibreWolf. 

Unlike open-source software, the source code of proprietary software is not available to the user. This means that hidden functionality such as telemetry and malware could be hidden inside the program. However, this allows for the developers to sell the software. (Although, there is some open-source software that is not free-of-charge).

The difference between open-source and proprietary software is an important distinction to make, since the two philosophies are fundamentally different and seek to achieve different goals. However, the most popular software in the world is still proprietary software, although some open-source projects dominate specific niche use-cases, such as Linux in servers.
assistant
How significant is open-source to our society?
user
Open-source software is significant to our society as the world runs on open-source. 

For example, the majority of computers in the world runs on Linux, an open-source OS kernel, including the overwhelming majority of servers as well as Android phones.
assistant