The conch serves several functions within the text. The first is that it brings the boys together. Piggy and Ralph find the conch and once Piggy explains how to blow it correctly, Ralph sounds a loud blast that brings the boys out of the jungle and the choir from down the beach.
As the boys discuss what to do and elect a chief, they also decide that they will use the conch to decide who gets to speak.
As such, the conch begins to symbolize rules and civilization. Particularly because Ralph and Piggy are very serious about maintaining rules and order, the conch begins to symbolize those things. Particularly because Ralph uses it to call meetings, the conch is important.
When Roger tumbles the boulder down on Piggy and kills him, he also destroys the conch. This signals the end of any sort of civilization on the island because the means of maintaining order has been crushed.