While trying to jailbreak your iphone 3G / 3Gs using redsn0w you probably would have noticed the option saying verbose boot
Well pretty simple, verbose boot means that iPhone will show you the text version of the boot and not the fancy logo while all the boot sequence works at the back end. It will show many text lines showing what the iPhone boot loader is doing right now.
Hopefully you got the idea now what the verbose boot is! :) (specially for iPhone verbose boot option)