Lord of the Flies, by William Golding, is a symbolic novel, and one of the primary symbols Golding uses is a conch shell. Ralph discovers the shell and retrieves it, but it is Piggy who understands how it might be used to help keep order on the island. He teaches Ralph how to blow it, and soon all the other boys respond and begin to gather.
The conch is a powerful symbol of order. When the boys decide to elect a leader, they do not choose Jack (who is already a leader) or Piggy (who has made the only efforts to lead and organize). Instead they choose Ralph.
None of the boys could have found good reason for this; what intelligence had been shown was traceable to Piggy while the most obvious leader was Jack. But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.
The conch works effectively for awhile, as the boys routinely respond its call and respect the rule that whoever holds the conch has the floor; eventually, though, Ralph is afraid to blow the conch because he is afraid no one will respond and they will all be forced to admit that there is no more order on this island. Just as the shell has begun to show the effects of time, sun, salt, and water, so the power of the conch begins to fade.
In the end, when Piggy confronts Jack and the other savages, he holds the conch in a desperate attempt to maintain some semblance of lawfulness and order on the island; however, Roger drops a boulder onto Piggy, and both he and the conch are crushed. When the conch, symbol of order, is gone, so is order itself.