Here’s a non-technical explanation of port forwarding:
Your home network is like a small community. Normally, the entrances (the ports) to your home network are closed to anything from the Internet that tries to control anything on your home network. These entrances are kept closed by a guard (your router).
When you use the port forwarding (sometimes called port mapping) feature to allow access your network, you are allowing network traffic to enter using that specific port number (think of it as an entrance) and going only to a specific device (in this case, your Slingbox). Your router opens one of these entrances for SlingPlayer to use. Whenever you watch your TV while you're away from home, SlingPlayer can connect directly to your Slingbox using that specific port. In other words, it is a specific connection, between specific devices, using a specific port. You are not opening any additional ports on your computer, for example. | 
