When you forward a port you are making a computer on your local network accessible to computers on the Internet. The computer does this by using a mediator, the router. Your local computer asks the router to get information from a site. Once the information is gathered the router checks it and then sends the requested information on to the correct computer on your local network. If you use port forwarding the information is gathered quicker because it doesn't need to make as many stops on its way to your local computer.Common uses for port forwarding are:
One way to look at port forwarding is a massive multi-lane freeway with lots of stops along the way. When you forward a port you are telling the router to bypass all stops on this particular port. It needs no checks because you've told it exactly where to go and that's it's perfectly safe to do so. A port would be a lane on that massive freeway. There are 65,536 different possible software ports. Each program on your computer that uses the Internet is designed to run on a specific port. The first 1,024 ports are all fixed. This is an industry standard so computers can communicate wherever you are located in the world. For example port 80 deals with HTML pages, port 110 is used by POP3 email and so on.
Before we start you need to know that every computer on your local network (including your router) has it's own unique IP address due to a technology called NAT (Network Address Translation). This technology is essential so your router can find and communicate with each wireless device on your network.
Every IP address is divided into ports (or lanes on a freeway). If you want to send data from one computer to another the first computer's IP address uses a port and sends the information to a port on the second computer. When you want a computer to connect to the Internet, the request needs to go through your router. First, the router receives the request from the computer. Your router takes note of which device on your network asked for the information. Then the router turns around and begins accessing the Internet. When the router does this it uses an external IP address, 188.8.131.52 for example. Every simple Internet request coming from your local network will use the same external IP address. Once the request has been answered, the router intercepts the incoming "packet" or request and acts as border security. It then determines if it's alright to continue. If it is it then sends it on the correct computer using that computer's LAN or internal IP address.
Before you can begin to forward a port, the device you are forwarding ports to may need to have a Static IP Address. Some router's require the computer you are forwarding ports to to have a non-changing IP address. If you think your router is one of these check out this link on how to create a static IP address.
That's it, you should be ready to go. Good luck with forwarding your ports!