D.O. stands for Designation of Origin (in catalan: Denominacio d’Origen), which essentially refers to the area where a wine comes from. Simply put, a wine is related to a D.O. in the same way a citizen is related to his/her country. Every single D.O. has its own land and climate characteristics that make its wines different from the rest. Every D.O. is governed by an institution (Consell Regulador) whose members are elected from the wineries that compose that specific D.O. and its mission is to ensure that the best practices and quality procedures are followed by all of its members.
There are 12 D.O.’s in Catalonia. You can locate them on this map of Catalonia: