Who was the "unknown citizen" in Auden's poem, and why was he called this?
Auden's poem "The Unknown Citizen" is written in the form of a government report about a citizen who is, in death, commemorated by the State but remembered only by numbers rather than by his name. The idea of the unknown citizen is usually understood to refer to the tombs of unknown soldiers which were erected after the First World War to represent all those lost in the conflict whose bodies were not found and whose names were not known. Auden's suggestion here is that this practice actually erases our individuality—and that the practice of modern life in general is making us lose our identities.
The unknown citizen, then, is an ordinary man who fitted perfectly into society during his lifetime. His employers were "satisfied" with him, and his views were in keeping with what was expected. He had friends, was insured, got married, had children, and did everything in the "normal" and "right" way. However, in death, he is nothing more than a statistic. The state commemorates him in the same way as it has commemorated, presumably, countless other people who also did everything in the right and normal fashion and contributed the right number of children to the population. Auden's commentary indicates that governments do not care whether people are actually happy: they want to reduce people to machines, which function according to pre-determined guidelines. But this is not really how people are, and it is dangerous to treat humans, who are individuals, in this fashion. If we do this, we will all become "unknown" in death.