Plain English Meaning
It is the name of a particular navigation station on the ground at Grand Junction, Colorado. Aircraft tune to it to find out which direction they are from the station and how far away they are. The letters JNC are its short identifier, the way an airport has a three-letter code.
Full Definition
A specific ground-based navigation facility located near Grand Junction, Colorado, identified by the three-letter ident JNC. It is a VORTAC, meaning it combines a VOR (which provides bearing information) and a TACAN (which provides distance information usable as DME by civil aircraft). Pilots and procedure designers reference JNC by name when it is used as a , a reference, or a on in that region.