A scar generally refers to a mark, blemish or indentation left on the skin or an object after it has been damaged. In the case of the island in Lord of the Flies, it refers to the mark left by the planes' fuselage when it crashes after being shot down. It is later washed away by the sea.
The first reference to the scar is found at the beginning of chapter one when Ralph is introduced as "the boy with fair hair":
All round him the long scar smashed into the jungle was a bath of heat.
The scar is quite extensive, and the word "smashed" indicates a violent encounter that caused damage. The story is set during a war, and the boys have been evacuated when their plane is damaged. The pilots are killed and the plane crashes onto the island. Most of the boys (as far as we can ascertain) survive. Ralph appears to be quite pleased, and when Piggy sees him he is standing on his head.
In the middle of the scar he stood on his head and grinned at the reversed fat boy.
When the two start speaking Piggy attempts to explain what has happened and provides a brief explanation of the crash and how they came to be there:
He looked up and down the scar.
“And this is what the cabin done.”
Golding's numerous references to the scar emphasize its significance. The scar is symbolic of a disruption in nature. The peaceful, calm and probably pristine nature of the island has been damaged by the sudden and violent arrival of humans. It suggests that man's intervention causes harm.
The presence of a scar also suggests that the damage is permanent.The scar will never disappear and will become an indelible indication of the harm that was done. Although the forest and plants will heal, there will always be evidence of what happened.
Furthermore, the scar foreshadows further disruption and destruction. The boys, for example, destroy a large section of the forest when their first signal fire goes out of control. At the end of the novel, a fire rages on the island and, ironically, becomes a signal for rescue. The boys are saved from their own destruction and that of the island. Other examples of destruction to come are the bloodthirsty hunts conducted by Jack and his hunters; the boys' general loss of civilized behavior; the adoption of savagery; and, eventually, the deaths of Simon and Piggy.