I'd describe it as a fuzzy surface condition in the barrel, be it erosion or corrosion, that covers the metal. Frosting is not serious enough to effect accuracy and should clear up after shooting some jacketed bullets, and is minor enough to clear up without leaving pitting.
Please note that it is at least my experience that most Luger barrels, be they frosted, pitted, or clean, seem to shoot just fine, at least as good as most old shooters can shoot them. Unless it makes sideways holes, worry more about feeding, extraction, etc.
dju
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