In Lord of the Flies by William Golding, the conch itself is a metaphor for law and order.
The entire novel is a kind of allegory, showing the flaws of civilization through the story of a group of schoolboys stranded on a deserted island. Shortly after the boys crash on the island, they find a surprisingly intact and beautiful conch shell. The boys decide that whoever holds the conch will be the sole speaker, thus establishing a sort of diplomatic procedure for governance. The conch starts off with Ralph, who represents a positive, just, and democratic form of government.
Throughout the novel, the conch remains a symbol of the order the boys initially establish. This is brought to a head when the conch is smashed, thus signifying the collapse of order as the boys descend into murderous, hedonistic chaos.
The choice of a conch to serve as this metaphor makes sense for a number of reasons. Practically, the conch is something that would be found on an island. Conches have also been historically used in ceremonies and to call out to people (as a kind of horn). Finally, the conch itself is both natural and fragile. It is something to be protected as it is so easily broken, and it is, of course, eventually shattered.