In chapter eight of Lord of the Flies, Jack and his fellow hunters are hunting for pigs. They successfully kill one and then decide to leave some of it as an offering to the beast that they imagine is stalking the island. The offering takes the form of the pig's head mounted on one end of sharpened stick. The other end of the stick is jammed into a crack between two rocks. Jack then declares, "This head is for the beast. It's a gift."
The pig's head mounted on the stick is a strange and gruesome sight. Golding describes it as "dim-eyed, grinning faintly, blood blackening between the teeth." Simon begins to imagine that the head is "grinning" at him, and then he begins to imagine that the head is talking to him, telling him that he is "a silly little boy" who should "run away." The head is then surrounded by flies and thus comes to be known as "the Lord of the Flies."
Although the Lord of the Flies is literally a pig's rotting head mounted on a stick, it also carries significant symbolic meaning. The Lord of the Flies symbolizes the fear that starts to take over and eventually overwhelm the boys on the island. This fear really comes from within themselves, but it is easier for the boys to understand if they can attribute it to a physical, external object. When the pig's head seems to taunt Simon, telling him that he is "a silly little boy," and that there "isn't anyone to help" him, this is really Simon's own internal voice. Indeed, the pig's head even says to Simon, "I'm part of you." The "Lord of the Flies" exists within the boys. It is the physical representation of the fears, anxieties, and doubts that erode the boys from within.