What is portforwarding

You need to login to your router and go to firewall or virtual servers.  In there you can setup port forwarding.  The goal is to forward external (internet) traffic to a specific internal machine (your PS3).  You do this on a per TCP/UDP port basis and must specify the appropriate external TCP and/or UDP port and internal TCP and/or UDP port.  Sounds harder than it is.
Or you can setup your PS3 in your router's DMZ (Demiliterized Zone).  This sets up the router to forward ALL internet sourced traffic to a specific machine (your PS3).  This is the easier way to do it so you don't have to worry about specific TCP/UDP port numbers.