What is port forwarding?
Port forwarding is a way to expose your computer from private network to public (internet). But, let's start with why: if you want someone to connect to your computer in order to play games together or remotely control your computer or host an FTP server, etc. you need to tell another person your IP address, so it can be used for connection. If you have a public IP address, then that's good and you don't need to deal with configuring your router for port mapping. But, if your computer is located behind NAT and all you have is private IP address on a network, then you need to tell the router: what would happen if someone is trying to send data to you? To which computer in a private network this data needed to be sent? That's where the port forwarding steps in.
Port forwarding configuration
Almost every router nowadays supports options for port forwarding. All you need is to tell the
router is what internal port you want to map to external port. So, let's
suppose you want to host a game on your computer and you need to expose the port
2302 to the outside world. Now, all you need to do is to update router's port
mapping configuration with your private IP address and port where game is listening for
incoming connections and external port which will be used by others to connect to the
game server.
Check that port forwarding works
In order to make sure that the port is really exposed to the outside, you can use PingPorts, which automatically detects your IP address,
then enters the port number and clicks the Ping ports button. Wait a moment and you
will be presented with the result, whether the particular port is opened or closed.
Periodic monitoring can be enabled, so you can update routers port forwarding
settings and watch if PingPorts detects that port is opened.