Lord of the Flies is full of symbols, all of which Golding uses masterfully to reinforce his larger theme of civilization versus savagery. Since no literary figure can ever be a literal representation, you are probably asking for the metaphoric and the deeper symbolic represetations of the conch. At the beginning of the novel, the conch metaphorically represents authority on the island. The shell gives Ralph the power to summon the other boys to assemblies and meetings. In chapter one, the boys liken the shell to the megaphone used by the man at the airport, again reinforcing in their mind the notion of an authority figure.
As a figurative metaphor for authority, the shell also is a symbol that symbolically represents civilization, rules, and order on the island. The boys choose Ralph as their chief, and he institutes the rule of using the conch as a "hands up" like at school to signal who may talk without interruption at the tribal assemblies. Golding builds upon this symbolism throughout the novel, culminating in the conch's destruction at Castle Rock, signalling the end to all pretense of civilization on the island.
It is also possible to define these two figurative representations as a metonymy for authority and an extended metaphor for civilization. One thing these representations cannot be is literal.