Port-forwarding is a way of selectively allowing access from a WAN (Internet) to your LAN through your existing router firewall. Only the network protocol “ports” that you forward will be allowed access to a particular box on your LAN. For instance, if you want to enable only FTP access to your ReadyNAS, simply forward ports 20 and 21 on your router to the ReadyNAS IP. You can do the same for HTTP or HTTPS by forwarding ports 80 and 443.
Each router will have a slightly different method to allow this option. Here's a link that explains how you would do this with a lot of the routers on the market. This link courtesy of MrCyberdude.