The scar is the damage done to the jungle by the plane wreckage. It serves as a reminder of the boys' forced presence on the island, as their crash has permanently changed the appearance of the island.
It serves as a symbol for evil as well. The island was untouched by man before they arrived, but now carries a visible wound. The arrival of humans has brought a blight upon the land. Similarly, each human carries a "visible wound"- the ability to carry out evil. It reflects that we each carry evil within ourselves, which can be inflicted on the world around us.
Finally, it foreshadows the division of the boys. Just as their plane carved and scarred the jungle, the eruption of violence and savagery will carve and scar their fragile society.
The scar referred to several times in passing while Golding describes the island is the path that the crashing plane tore through the jungle. The first paragraphs mention the scar, but don't go into detail. The reader can imagine how large and devastating the path must look, but Golding understates it. Piggy explains that the plane went down in flames, causing tree trunks to fall as it crashed. Observing the damage to the landscape, Piggy says, "this is what the cabin done." The cabin of the downed plane was washed out to sea in the storm.
The scar is a subtle reminder of the trauma the boys have experienced, a topic which Golding leaves largely unexplored. Although the younger boys are plagued by nightmares and some of the older boys wistfully discuss their missing parents and caregivers, little time is spent describing the horror of the evacuation, crash landing, or possible loss of some friends or acquaintances in the crash. It seems likely that some boys died in the crash or drowned when the cabin was washed out to sea.
This scar on the landscape mirrors a scar in each of the boys' hearts. They have each suffered trauma, and they also have each been damaged by their associations with the outside world and by their contact with "mankind's essential illness." Just as the island is marred by the results of human conflict and violence (the nuclear war taking place in the outside world), so the boys are tainted with humanity, a depravity that they cannot escape because it is a permanent part of them.