In William Golding's Lord of the Flies, the fire symbolizes both hope and enlightenment. Specifically, it symbolizes the hope of being rescued and returning to civilization; plus, since the creation of fire was the first thing to help ancient civilization progress towards modernization, fire also symbolizes the boy's enlightment, particularly the enlightenment they held while still in civilazation.
It is in Chapter 10 that Ralph says to Piggy and Samneric, "Let the fire go then, for tonight." Prior to this moment, many barbaric things had happened, including the murder of Simon. At this particular moment in Chapter 10, Ralph, Piggy, and Samneric are trying to collect firewood, but Piggy says he can't due to his asthma while Eric complains of being too tired to continue collecting. At the point that Ralph tells them to "let the fire go," all four characters have reached a point of despair. Ralph sees the impossibility of only the four of them working to maintain the fire, while the other boys see the impossibility of collecting more wood, at least in the dark that night.
Hence, in telling the other boys to "let the fire go," he is actually symbolically portraying their despair. Both he and the other boys have given up on the hope of being rescued and of returning to enlightenment and civilization.