In ancient Rome, a proconsul was a man sent to govern a province — a part of the empire away from the city of Rome. He held power like a consul (one of Rome’s top officials), but he held it out in the provinces, in the name of the state. The pro- here is “in place of / acting as.” It is a history-classroom word. Though actually, later writers sometimes stretch it for a powerful official who runs a distant region as if it were his; that is a later colouring. For this sense, stay with the Roman governor of a province.