There are a number of possible interpretations of the conch shell's symbolism. Ralph discovers it on the beach, and Piggy suggests using it to make noise to signal for rescue from passing ships. Once it becomes clear that rescue is improbable, the conch is used by Ralph to call the boys together for meetings. In these meetings, whoever holds the conch is allowed to speak. In this way, the conch represents order and civility: it is literally the symbolic object used to denote speaking order and so encourages respect and civil engagement. But eventually, as tension grows and two different factions are formed, the conch is smashed.
Because it is Piggy's idea to use the conch in a useful way (Ralph merely found it beautiful), and because its destruction occurs as Piggy is accidentally killed, the conch also represents Piggy's part in the social dynamic: intelligent but physically weak, an outsider whose ideas help Ralph to emerge as a leader.
In Buddhist spirituality, conch shells represent truth and strength. The loss of the conch leads to a loss of cohesion, and Ralph's intentions to lead honorably are compromised.
It is worth saying that the conch shell also represents female energy; the shape and color of a conch shell has been compared to female genitalia, and the general qualities associated with women in society (cooperation, nurturing, equality, sharing, etc.) are largely absent from this group of boys left to fend for themselves. The conch is destroyed at a time when some of the boys, led by Jack, are indulging in manly pursuits (hunting, fighting, building bigger fires than necessary, painting themselves like warriors, etc.) and its loss denotes a loss of the feminine influence that may have been helping hold the social order together. Interestingly, Piggy is killed after the boys develop a taste for the wild pig meat hunted by Jack, leader of the rival faction that does not want to follow Ralph's leadership. Indeed, after Piggy's death, Ralph's position as leader (a position he tries to use to unite the boys and help them survive in a cooperative way) is threatened and his life is endangered.