Plain English Meaning
On this approach, you are not allowed to navigate using a system that figures out your position by measuring distances from two ground-based DME stations alone. There aren't enough DME stations in the right places to keep you accurate enough, so that method is off-limits here.
Full Definition
A chart note indicating that the procedure may not be flown using DME/DME-only RNAV equipment that requires an RNP value of 0.3 nautical miles. The procedure's design does not provide adequate DME ground-station coverage to support an RNAV system relying solely on two or more DME stations to achieve and hold lateral accuracy within 0., so that is for this approach. using or other RNAV sources may still be .