DND stands for Do Not Double. If your coupon says Do Not Double it will generally be only accepted at face value.

HOWEVER...If a coupon barcode starts with a "5", it will probably double if the store does double coupons. If a coupon starts with a "9", it will not double.

So...your next question is...why would they even put the language on the coupon in the first place? Manufacturers only reimburse the store for the face value of the coupon + a small handling fee. The store then picks up any cost of doubling a coupon. For example...
If Frys doubles a Cottonelle .50 coupon to $1.00, then the manufacturer will reimburse Frys .50 + a handling fee. Frys takes the remainder .50 as a "hit" to their profit margin.

So...why does a coupon need to say Do Not Double?
Well...My guess is that it has to do with clarity or legal matters....
The idea here is that the manufacturer is "reminding" the store that they will only reimburse the face value + a small handling fee, but will not reimburse the store for double the value. Like legal code...cover your bum sort of thing....

So...now you know!