Here are some synonyms:
* Authoritarian
* Commanding
* Domineering
* Haughty
* Overbearing
* Tyrannical
It essentially describes someone who expects obedience and acts as if they are superior to others.
Here are some examples of how "imperious" is used in a sentence:
* "The general's imperious tone left no room for argument."
* "The queen's imperious gaze swept over the assembled court."
* "He demanded silence with an imperious wave of his hand."