It is a lot like human dandruff and therefore usually harmless but there is a chance that it can be caused by parasites or skin infections so best to get it checked out by a vet, if the dog is showing signs of irritation in the areas of dandruff it is definitely a problem.
If not it can be helped by regular bathing (although it can be cause by dog shampoo not being washed off properly), enough for the individual breed/coat type, consult professional groomer for this or a better diet, again consult the vet about this.
I think first step is to go to the vets, that would be your best bet, good luck and I hope your dog is OK