I'm guessing you see it on an approach plate. It means that if your aircraft uses DME/DME to as the sole means to establish location you can't use that particular procedure. Not a big issue for most light GA aircraft.
Larger aircraft generally use a combination of ways to determine location. For example, my aircraft uses (in this order):
GPS/GPS
GPS
DME/DME
VOR/DME
VOR/VOR
IRS
Dead Reckoning using last know position, heading and speed.