Every computer has an IP address and a port for any internet traffic. Each computer on your network holds a different IP so that your router can locate them and direct traffic to the correct machine. These are internal IPs - only your router and your computers deal with them.
Your router also has its own IP, this is the address at which your router can be seen by the whole internets. When data passes between the router and the internet/your computer, the internal address has to be converted to the external address. This is called Network Address Translation (NAT).
A port is like the door to a tunnel. There are a little over 65000 different ports you can choose from. When you forward a port in your router, you allow the door to one tunnel to be open for one internal IP address. That means that you're only opening the port as much as you have to, reducing security risks.
What this means is that the internal IP address needs to always be the same (static) for the computer that the port is forwarded for, so you need to set static IP addresses.
Check out PortForward.com - Free Help Setting up Your Router or Firewall for instructions on forwarding ports for most routers, and how to set static IP addresses.