As pointed out by the first answer to your question, the Lord of the Flies is a pig's head on a stick, but Golding manipulates its literal portrayal--a dead pig's head on a stick surrounded by flies--to create a very figurative meaning for the reader. The image itself represents death and decay, a waste of life, so Golding uses that image to create a much deeper meaning in the text, using the dead pig's head as a front for the metaphorical Lord of the Flies.
"[The flies] were black and iridescent green and without number; and in front of Simon, the Lord of the Flies hung on his stick and grinned. At last Simon gave up and looked back; saw the white teeth and dim eyes, the blood--and his gaze was held back by that ancient, inescapable recognition" (138).
Golding's use of detail, from the color of the flies to the sharp contrast of the "white teeth and dim eyes," creates a horrible picture of destruction; his use of the name 'Lord of the Flies' alludes to Beezelbub, a name literally meaning 'Lord of the Flies,' taken from the Old Testament of the Bible for the devil, representing that deeper sense of evil, what Simon perceives with "ancient, inescapable recognition" (138). Simon's encounter with the Lord of the Flies is made all the more disturbing and grim through Golding's use detail and imagery.