The mountain in "Lord of the Flies" symbolizes hope and truth, according to Enotes. By reaching the top of the mountain, the boys gain hope of surviving their situation and realize truth of what the island is as new information is revealed and more challenges confront them.
As the boys progress toward the top of the mountain, their hope grows because rescue is much more possible if they reach their goal. Their objective is to reach the top of the mountain to start a fire to signal for help. Moreover, as they reach their ideal vantage point, they can discover the truth of the island and their situation.
When the boys finally start the fire, they realize that they must keep it lit — a hard truth that is shown when two of them fail. Later in the book, the boys are also attacked on the mountain, and they find that the mountain will not keep them safe from beasts. Furthermore, on the mountain, Simon finds out the truth about the lord of the flies and what the beast truly is. The longer the boys stay on the mountain, the more truths about the themselves and the island are revealed.