�Sir� in English is a form of personal address, more respectful than �hey, what�s-your-name�. Being knighted by a monarch also conveys it as an honorific (mistakenly called a �prefix� to a name) like Mister, Miss, Doctor… eg. �Sir James�
In the military, it�s also a form of address to a commissioned officer but when asked as a question �Sir?� it is a respectful way of expressing confusion about an order given by or statement made by a superior officer or it can be a subtle but nominally respectful way of disagreement with a superior officer through feigning confusion, sort of an implied �Sir, have you really thought this through?�