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.