Stands for STAndard NATO AGreement. Found here:.
A rather boring set of guidelines that sets standards for various aspects of NATO technology, including such exciting things as paint.
This is occasionally actually useful; the fact that all NATO nations' 5.56mm assault rifles are standardized to use the same thirty-round magazines (often called STANAG magazines) allows units to share ammunition in a pinch.
Except for the French of course, who insisted on making their FAMAS rifles incompatible with STANAG magazines. But it's not like the French are actually expecting to fire their rifles anyway is it?
Also all NATO stores are given numbers (NSNs). These are the distinctive 13 figure numbers found on most of the tools in your average Squaddies shed.