emuNAND is a method to emulate the NAND, it means it emulate the internal data of the Wii using a copy located on SD or USB.
There are two method to use EmuNAND :
real time redirection by cIOS. (USBLoaderGX is using this one. It doesn't have best compatibility)
No cIOS, all the Wii is emulated: neek or neek2o is using this method. It acts like a real unhacked Wii, but loading data from external device (IOS, Mii, Settings, Wiimote synch, Savegames, etc.)
using emuNAND, you can create a fake NAND structure (file, folder expected by the Wii, but on your SD instead of the internal NAND chipset) with a different region setting.
for example, if you create a NAND structure on your SD with a USA region, when you use this path as your EmuNAND, you will have a USA wii.
The advantage is that all you do is done on SD.
Your real NAND is never affected, and you don't risk to brick it.