this post was submitted on 06 Oct 2023
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Linux
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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Every computer has a bunch of ports (1-65535 if I recall correctly), each of which is a unique entity to which a single service can bind. In layman's terms, a port is a door that one service is able to answer when someone knocks. By convention, some ports have a specific associated service (80 = HTTP, 443 = HTTPS, 22 = SSH), but there are a lot that you can just use as you deem appropriate.
If you want a service (e.g. a web server) to be accessible, you have to run a service that binds to a known port (e.g. 80), and a client has to reach out to your server on that same port. A firewall sits between your service(s) and any potential clients, much like those steel security screen doors. If that's closed, nobody gets through on that port, even if a service is bound to that port and is listening for a connection.
As a general rule of thumb, you want your firewall to block as much traffic as possible without breaking something (I.e. blocking one of your public-facing services). If you don't run any services on your computer (web services, media servers, etc.), you can probably get away with blocking all inbound traffic. without any discernable impact.