Stupid question: I do not know how computers work and I cannot for the life of me grasp how seeding works.
Snov and Nitro have given nice answers about the torrent protocols for seeding, but allow me to add a little bit about the Internet networking required for seeding to be successful.
You may know what a proxy is: a webservice that obfuscates your IP by tumbling and passing your traffic through an intermediate address. The same consideration applies for traffic on your local area network (LAN) before it goes out to the Internet at large, i.e., your desktop, laptop, and cell phone (on wifi) have the same wide area network (WAN) IP address despite being different devices.
Without proper networking, external torrent clients can see your WAN address, and say "ok, I'll connect to your IP!", but then get lost when they arrive at your router, because your router doesn't receive explicit instructions to send the traffic to the device that is running your torrent client. The connection is broken, your torrent client is considered "unconnectable," and no seeding occurs.
The solution to this problem is called "port forwarding" and happens at the router level. This is also known as a reverse proxy. The bittorrent protocol directs connections to your WAN address as well as a port, and the router is able to interpret the port number to identify the device that should receive the torrent traffic. Let's consider an example where your WAN address is 223.556.308.762, and your are torrenting using your desktop with LAN address 192.168.1.10. In the settings of your torrent client, you can specify a port number that is attached to your WAN when your torrent client announces that you have the files and are willing to seed. Let's go with port number 6881. When connecting to other people's torrent clients, your client will say "hey guys, connect to me at 223.556.308.762:6881!" Then, in your router settings you make a rule that tells your local network "ok, all connections that head to 223.556.308.762:6881, please send that to the desktop located at LAN address 192.168.1.10:6881." Further, once that traffic reaches your desktop at 192.168.1.10:6881, the computer will recognize the port number of 6881 and say "ah, I recognize the 6881, this traffic belongs to the torrent client!" Now, incoming torrent connections are possible, and you can seed your files.
I think the imagery of an air traffic controller is apt for explaining this concept further. An empty cargo plane (connection request) is headed to your airport, Sneedsville, originating from Chucksberg. The plane is going to pick up freight (data) and fly back to Chucksberg. Your job is to get the plane landed (receive the packet via WAN), direct it to terminal A (redirect to your desktop), and tell the plane to taxi to gate 6881 (specify which program wants the data). At this point the plane knows how to get back home and you can let it do its thing on its own.
Format: (a) airport instructions, (b) torrent equivalent
Step 0a. Sneedsville announces the airport location, and that it is willing to receive the plane, load cargo, and send it back. You don't care what terminal or gate the cargo plane used to depart Chucksberg, not your problem.
Step 0b. Your desktop (LAN address 192.168.1.10) is running a torrent client, which is listening at port 6881. The torrent client announces to other clients that it can be reached at your WAN address:port number, in this case, 223.556.308.762:6881.
Step 1a. The Chucksberg cargo plane has arrived in your airspace unaccosted by sky-jannies (Cogent, cloudflare, Liz Fong-Jones). Sneedville ATC lands it on the runway belonging to Terminal A.
Step 1b. The incoming packet arrives at your router, 223.556.308.762, carrying the identifier of 6881. Your router's rules recognize that this traffic belongs to your desktop and sends it to 192.168.1.10.
Step 2a. Sneedville ground control takes over from ATC and directs the plane to Gate 6881. Once at gate 6881, the plane loads its cargo and does the process in reverse. It's 5pm and you want to get happy hour margaritas. You figure that the plane can remember the steps in reverse, so you punch out and hit the bar.
Step 2b. Your desktop (192.168.1.10) identifies the port 6881 as a connection with your torrent client and sends it there. Your torrent client sends the packet of data to the requesting address using the now open connection without any further action on your behalf.
The step-by-step instructions for establishing port forwarding vary with your router's software and computer operating system used. There are plenty of guides online that explain the steps in detail, therefore, even the uninitiated can set up port forwarding in ~15-30 minutes. Don't let the idea of port forwarding scare you, it's not that hard and the result (the ability to survive on private trackers) is well worth it.
Finally, don't give up hope if you use a network where you are not able to tinker with the settings (e.g. managed networks on college campuses, building-wide internet, etc). In this case, you can use a VPN that allows port forwarding to simultaneously hide your IP address from copyright agents and also establish connectivity. If this applies to you, identify a VPN provider that you want to use and then follow their specific instructions for setting up their VPN with port forwarding. I have experience with AirVPN as the VPN provider and the docker container binhex/arch-qbittorrentvpn as the torrent client and could assist in troubleshooting if necessary.
As always, I am receptive to corrections if anyone spots errors!
