What am I supposed to put in the box that says pattern? Sorry, I'm really stupid and it's probably really obvious, but could someone please explain without any technical talk? I know absolutely nothing about computers!

Sorry, you'll need to know quite a bit about computers before you'll get FoxyProxy working. It's not for novices. There is quite a bit of documentation at http://foxyproxy.mozdev.org on how to use it, but all of that documentation assumes a certain level of knowledge already.