The poem itself is pretty direct; the increase in technology in the world, and the ability of government to be able to "track" a man's life based on that data gathered from that technology, is all satirically commented on by Auden.
The title and dedication itself can be seen as a kind of symbol; instead of naming the man, he is known as "the unknown citizen", symbolizing how his society doesn't need a name for him, just a number: "JS/07/M/38/". The lack of name and the number is a symbol of the alienation of the individual from the society, the lack of humanness in technology. An uknown citizen symbolizes the masses of men who go to war to lay down their lives for their country (akin to "an uknown soldier).
The monument erected in the citizen's honor is a symbol of what this type of society values most: a citizen who follows the rules, follows their opinions, and never causes trouble. According to their stats, this man fit the bill, so they erected a monument. All of this implies "do what you are expected to do, and we will honor you" and symbolizes the deindividuation of people expected by that society.
The various representative organizations presenting information on the man represent the cold, distant nature of technology and statistics, and how they cannot possibly reflect who a person really is. As the last lines state, "Was he free? Was he happy? The question is absurd." They didn't know him at all, and didn't care.