Planetary governors are Imperial agents who represent the Empire's authority on a single world. Usually that representation extends to an entire system, giving him jurisdiction of all the planets orbiting a single star. A governor has command of all Imperial troops garrisoned on his planet.
This command extends only nominally to the political and intelligence arms of the military, which operate as if a governor's orders were well-intentioned suggestions. The Imperial Navy is supposed to consult with a governor whenever they are on maneuvers or engaging hostiles in his system, and to carefully consider the advice the governor can provide. The Navy routinely ignores this and most other forms of protocol when dealing with planetary governors, unless the governor is well supported by a Grand Governor. The Navy considers governors to be too concerned with the welfare of their planet to be of much use in a crisis.
Governors are appointed by the Emperor's advisors, although a few are just long distance communications approvals of candidates the Emperor has chosen. Governors are rarely native to the planet or system which they govern. Rash appointments without regard for local sentiment can generate resentment. Instead, and outsider is appointed to govern a world, thus bypassing local sentiment completely.
Governors are expected to let a planetary government run its own affairs unless the local course of action puts the planet in conflict with Imperial goals. The governor is then expected to step in and maneuver the policy back toward a direction more acceptable to the Empire.
Imperial policy exists in two different forms for a governor. There are the generally stated policy goals which are distributed by the Diplomatic Service, and there are direct orders received from the Grand Governor. In case of a conflict, the Grand Governor's orders are to be followed, as a Grand Governor presumably has superior knowledge of the specific Imperial policy currently being employed in a particular sector.