well. bit tricky to answer.
basically portforward.com is a web site that explains how to forward the ports of your firewall.
Not sure what you do and don't know about modems, routers and firewalls
so i'll keep it simple.
A fire wall ( albeit it in your computer (win xp) or in your modem/ router) is a barrier that protects you from unwanted attacks via the internet.
The wall itself is split into chunks ( bricks) known as ports. if you do want to allow access to other people then you have to open a port or ports in order for them to "see" your computer. There are some ports that always need to be open...for example port 80 is for http ( the internet) if this was closed you wouldn't be able to surf the net.
port forward means that if a call or request comes in to a port you direct it to the PC and piece of software running and "looking" at that port.
Example.
I have 2 PC's at home. one of them runs an FTP server. ( a file transfer program) I have set up a port for FTP. 1001. If somebody that knows my IP try to log into that port with an FTP program, my router goes "Heh thats an FTP program trying to gain access on port 1001 so i'll direct it( forward it) to the PC thats running the FTP server.
Now, as is the case with torrents you need to allow people to download "from" you at the same time you download the file you want.
Thats why it's called sharing.
So depending on whether you have a modem/router with firewall or are using windows firewall you need to "allow" this to happen. by opening the port of your torrent program.
People who upload as well as download are always treated better. better speed.
Now if you use for example utorrent1.6 or later you need to open the port it uses. In the preferences of utorrent you can specify which port it should use. You now have to make sure the same port is open in your firewall. Otherwise other P2P users can't "see " you.
In windows firewall it's called exceptions. In Netgear routers it's called services.
Go to portforward.com to find insructions on how to set up your router/modem to open a port.
If you get stuck just let me know and i'll try to help. i will need to know what modem router you have and if you have windows firewall running or not.
Good luck