The fire is mentioned by Ralph in chapter three when he mentions, they would need the fire to be rescued off the island. It is also a form of communication to the outside to catch someone’s attention to help rescue the kids off the island. Ralph says "If a ship comes near the island they may not notice us. So we must make smoke on top of the mountain. We must make a fire.” Jack and Ralph created the fire by using Piggy’s glasses which Piggy did not agree with because he wasn’t happy about the way it was being used but at the moment it was the only possible way to start the fire in order for someone to come to their rescue