When you see one particular object pop up again and again in a story, that's a good indication that the item is a symbol, meaning that it has more significance than just its immediate function as a thing. It stands for an idea. (Usually more than one.)
When Ralph first finds the shell, Piggy gets excited and says "I seen one like that before. On someone's back wall. A conch he called it. He used to blow it and then his mum would come."
Realizing how useful it is to make a loud sound with the shell as a means of calling themselves all together, the boys use the conch for that purpose. They also use it as a discussion tool: since they make too much noise and get nothing accomplished when everybody talks at once, they make it a rule that whoever is holding the conch gets to talk while the others listen.
Of course, as civilized behavior starts to break down among the boys and they turn into wild beings, a shout of "I got the conch!" doesn't help establish order any longer. The beautiful conch eventually gets shattered into tiny pieces--right at the moment that the always-civilized Piggy is murdered.
So, throughout the novel, the conch shell remains a symbol of authority (because whoever is holding it is in charge) as well as unity and civilization (because it calls the boys together and keeps them in order). When the conch is destroyed, so too are the last bits of reason, cooperation, and civilized behavior among the boys.