DOP is a legal designation in Italy. Other countries have similar designations. The term "champagne" is one example. If the wine is not from that region, it cannot legally be called "champagne"; it must be labeled something else, like "methode champenoise" or "champagne style" or some such name.
The same is true of foods like cheeses, Parma ham, etc. in Italy. The food must come from a very particular region which is controlled by a licensing authority. Sometimes this is a co-op of producers. The authority inspects the manufacturing of the food product to assure it's made according to very particular, traditional standards and not with some shortcuts or adulteration that mimics the "real thing". So, San Marzano tomatoes with "DOP" on the label should be the real thing, grown and packed in that region.
I've bought them in little Italian groceries for as little as $.99 a can. (For the benefit of the growing league of CT Wisconsinites, that was at Glorioso's in Milwaukee and Tenuta's in Kenosha.)
