Ralph uses the conch shell in two ways. First, he uses it as a signal for others to gather for a reason. When he blows it like a horn, other boys come running and and assemble themselves together. Secondly, once the meeting begins, whoever holds the conch shell is permitted to speak. This ensures that meetings are conducted in an orderly fashion, and that everyone gets a chance to speak their piece without fear of interruption.
Piggy seconds the use of the conch shell for order, as he is overly concerned with having a list of who is on the island, as well as knowing what everyone's doing. Ralph, the designated chief during the first part of the novel, tries to allow everyone to have a say, so the conch shell allows him to do just this, without having to yell and use his authority too often.