Ralph uses the conch shell to call a meeting. If you blow into one it makes a very loud, distinctive noise, just what you need to bring an unruly mob of boys to order. From the moment that Ralph first uses the conch shell to call a meeting, it becomes a symbol of the kind of rule-based order that he wants to establish on the island. It's also a symbol of authority, which it confers upon whichever boy happens to hold it.
But Jack's not interested in rules and stability. He wants to be king of the castle, the undisputed dictator on the island. And he wants to establish his dictatorship on the basis of cruelty, domination, and control. For that reason, the conch is a threat to his power; so long as it's there, it will act as a reminder of the kind of civilization that Ralph wants to establish. It comes as no surprise when the final destruction of the conch shell later on in the story represents the end of all Ralph's hopes and dreams. The boys have been degenerating into savagery under Jack's leadership for some time, but so long as the conch was around, it acted as a symbol of hope for a better community. But once the conch is dashed on the rocks, any remaining hope is dashed with it.
The conch is a type of shell that Ralph spots lying in the lagoon. He recognises that it is a shell, but it is Piggy who identifies it as a conch. Piggy tells Ralph that the are very valuable shells and that you can also blow in them to make a sound. It is Piggy who comes up with the idea of using the conch to summon the other boys on the island, and instructs Ralph in how to use it to make a sound.
Of course, the conch becomes deeply symbolic in the rest of the novel as a symbol of civilisation. It is used as a method of keeping order in the meetings, as the only person who can speak is the person holding the conch, during which the rest of the boys have to be silent and listen. Of course, this is an order that is short-lived, as Jack and his hunters deliberately defy these rules, symbolising the descent into disorder and chaos that characterises the rest of the novel.