If you have to ask you will not need this. That being said, an NMS files takes an actual rom name and converts it to something more user friedly for the front end to display in its game list. It must be supported by the front end.
a typical entry would be something like:
Donkey Kong|dkong
with the part before the "|" being the rom name and the part after the "|" being the user freindly part.
John