It’s the one thing when I’m configuring things that makes me wince because I know it will give me the business, and I know it shouldn’t, but it does, every time. I have no real idea what I’m doing, what it is, how it works, so of course I’m blindly following instructions like a monkey at a typewriter.
Please guide me into enlightenment.
All these answers read like they’re written for comp sci students rather than a general audience. Let me give an ELI5 (more like ELI12) a shot.
Ports are just numbers. They aren’t physical pathways or doors or windows or anything like that. A better analogy is a street address, like an apartment number. Your IP address identifies your computer (apartment building), and the port identifies the program on the computer (the apartment). When a program needs to talk to the internet, which is very similar to sending a letter, it hands a packet/letter to your computer and your computer assigns the program a port number. It then puts that number on the return address of the letter so that the recipient knows where to send the response. The computer remembers that port number is associated with that program, so when it gets an incoming letter with that number, it gives it to the program. After the program is done talking to the internet, the computer frees the port up to be used by another program.
Ports are “closed” when there is no program associated with them. Any incoming letters are ignored because they have nowhere to go.
Ports are “open” when they’re associated with a program. This happens automatically when programs send outgoing letters, or you can manually open (or “forward”) ports by telling your computer/router what the port should be associated with and that it shouldn’t use the port for something else.
ELI5 over.
The internet is networks on top of networks on top of networks, so your computer will have an IP and assign a port number, then your router will remember that and change the address on the letter to its own IP with a different port number, then that process repeats a few more times until eventually it reaches its destination. You don’t have to deal much with your computer’s internal network, but occasionally you have to deal with your router’s by opening/forwarding a port because it has a NAT that has to deal with all of the devices on your network. Forwarding the port just tells your router to always send incoming letters with that port number to a specific device.