the Lord of the Flies represents the evil within us (in the book). the Lord of the Flies basically tells Simon that there's no escaping him because he's inside us all. this is communicated through a strange daydream Simon has due to lack of food and water, where he talks to the head of the pig and realized the grim truth of who/what the real beast on the island is: themselves.
in my book, this scene is on pg 137-138
★ "Lord of the Flies" literally translates to "Beelzebub" in Greek meaning the Devil