Ahh, someone else would have to explain flashing.. I'm not either and I don't want to be the cause of someone breaking their device sorry.
I can tell you that rootfs is the operating system, and emmc is the 27gb storage which I believe also holds settings for apps. You need to flash them in the correct order (not sure which way round though) and not power on the device in between flashes. I hope this helps a bit. I'm sure a how-to would be easy to find with a search either on here, or google =)