Lord of the Flies, by William Golding, is a symbolic novel set on a tropical island, so it is not surprising that Golding uses a conch shell as one of the primary symbols of the book.
The story of the conch actually begins before Ralph blows it. Though Ralph is the one who discovers and retrieves the conch shell, it is Piggy who understands its significance. He has to teach Ralph to blow it, so from the beginning the conch is connected to both Ralph and Piggy.
Of course, after Ralph blows the conch and the boys have a meeting, the conch becomes a symbol of order, rules, and civility. From this point on, whatever happens to the conch reflects what happens to the boys.
Despite no demonstrable leadership skills, Ralph is elected leader:
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.
Even Jack respects the conch at first (except when Piggy has it), but soon the shell begins to do what shells do when they are exposed to salt water, sun, and sand: it begins a slow disintegration. This matches the disintegration of civilized behavior on the island. Piggy clings desperately to the conch and the order it stands for, but it is a lost cause.
Eventually things get so bad that, though Ralph can blow the conch to call a meeting, he does not want to do so.
"If I blow the conch and they don't come back; then we've had it. We shan't keep the fire going. We'll be like animals. We'll never be rescued."
Ralph clings to the belief that he can keep his power as long as he holds the conch, but it is an illusion.
After Jack and his tribe come to steal Piggy's glasses, Piggy grabs the conch and finally wants to confront Jack. He has never wanted to do this before because he knows Jack hates him, but he has had enough and makes Ralph take him up the mountain to Jack and the others.
Piggy makes one last plea for order amid the chaos:
“I got this to say. You’re acting like a crowd of kids.” The booing rose and died again as Piggy lifted the white, magic shell.
“Which is better—to be a pack of painted Indians like you are, or to be sensible like Ralph is?”
A great clamor rose among the savages. Piggy shouted again.
“Which is better—to have rules and agree, or to hunt and kill?”
The answer to Piggy's question comes in the form of more violence. Roger levers a boulder from above which smashes Piggy and shatters the conch.
The conch is gone and savagery reigns. The first thing that happens is that Jack attacks Ralph with his spear, an indication of the violence to come. While the conch was just a shell and incapable of literally maintaining order, it was a symbolic image of order and civility on the island. When it is gone, chaos reigns.