The signal fire was originally lit and kept burning in case a ship comes past the island and will rescue him. The signal fire originally symbolizes hope - hope that the boys will be rescued. Towards the end of the novel the signal fire changes. This represents C - a gauge of the boys desire to return to civilization. The boys let the signal fire go out. They are more interested in hunting and killing Ralph than being rescued so they light a fire to try and flush Ralph out from his hiding place. Ironically this is the fire that catches the attention of a passing ship and the boys are rescued.
