Plain English Meaning
A ground-based navigation station has been turned off or is out of service. Any route or approach that relies on it cannot be flown until ATC issues a replacement route or another way to navigate is approved.
Full Definition
The planned or unplanned removal from service of a VOR or VORTAC ground navigation station, which makes any airway, route, instrument approach, or procedure that depends on that station temporarily unusable and triggers the need for a substitute route or alternate means of navigation.