PDO stands for "Protected Designation of Origin," a product label established by European Union law in 1992 to protect the names of regional foods.

The designation is meant to keep producers of regional products safe from unfair competition and rivals who may try to pass off inferior knock-offs using prestigious labels. The notion of PDO is tied to the idea of terroir and the importance of using raw materials and production methods from a certain region as a measure of authenticity.

PDO laws are designed to protect cheeses like Parmigiano-Reggiano and Champagne, the world's most famous sparkling wine. But the laws also extend to hams, olives, sausages, beers, vegetables, and regional breads. 

The EU has bilateral agreements with some countries to help enforce these laws. The U.S. is not one of them, which is why you can find generic American-made "parmesan," "champagne," "camembert," "prosciutto," "feta" and other protected products on the market.

Some regional trademark protections do exist in the U.S. for products such as Vidalia onions, which must come from a designated region around Vidalia, Georgia. But keep in mind that brand names don't necessarily indicate where a product is made. Philadelphia Cream Cheese was never made in Philly.

A list of PDO-designated products can be found here.