NAT means it blocks incoming (can mean outgoing also) for certain ranges of ports. So port forwarding means telnetting into the device via your browser, & making special rules to allow passage through particular ports. Port forwarding is very common for games & video conferencing, both which usually require many ports to be opened. LW & FW only need a single port opened. Or one for each if you use them at the same time.

Definition of port forwarding: http://en.wikipedia.org/wiki/Port_forward