DOP stands for Denominazione di Origine Protetta, Italian for Protected Designation of Origin, or PDO in English. In France you'll see AOP, and in Spain DOP again. It's the same idea: the cheese's name is legally protected.
To carry the label, a cheese has to be made in a defined region and follow a written specification for how it's made, including where the milk comes from and the traditional methods used. Producers are inspected to make sure they stick to it. That's why real Parmigiano Reggiano, Roquefort and Manchego can only come from their home regions.
What it doesn't mean: that the cheese is automatically better than one without the label. It's a guarantee of origin and method, not of taste. But it's a handy way to know you're getting the real thing.
Related: Is cheese vegetarian? · What is rennet? · What is the difference between raw milk and pasteurized milk cheese?