From outside your network to access.
Port Forwarding is an option available in almost all home routers. Because your server PC is likely behind a router (your home network is a Local Area Network [LAN] with a router connecting it to the Internet), your PC's IP address (the number that other PC's use to talk to the server PC) only works within your local network (all the PC's connected to your router). In order for the outside world (Internet) to talk directly to your server, your router needs to let the two communicate. Normally, the outside world (Internet) would talk to your router, and your router would relay that message to your PC. This works great for browsing, but not for serving.
To open this communication channel, we need to look at ports. Ports are "channels" that different programs use to talk on. Some common ports:
21 - FTP
80 - HTTP
5900 - VNC
29070 - Jedi Academy (used to host a JKA server)
There's tons more (like 65 thousand of them lol) but that's not the point. The point is, you need the port on your Internet IP address to be connected directly to the port on your server's local IP address.
