A power structure refers to a system of government within the world the writer has created.
A description of a power structure would capture the way in which power or authority is distributed between people within groups such as a government, nation, institution, organisation, or a society. Power structures are systems that are used to exert control over a government, organisation or resource.
Who holds power within this world? I.e., King, Queen, government, high council etc.
In order to develop your power structure, it’s often conducive to conduct research into real-life systems of government. You can use aspects of reality as a basis for your own system.