the term "flash" is basically referring to your ROM read this and it will give you the idea i know it looks like a lot but there are thousands of things out there to read up on so check this out:
http://forum.ppcgeeks.com/showthread.php?t=17473
flashing is a way to update ROM/radio version and the post I gave you will tell you exactly how to do so...If that's what you are interested in.