What is the signifcance of the scar in "Lord of the Flies"?
The scar is actually the place where the plane ripped across the island as it was crashing. Symbolically, it represents the injuries suffered by the island as a result of human habitation. When the boys arrive, the island is an idyllic tropical paradise. The wreak havoc on the environment by eating all the fruit, using the land as a public toilet, and mercilessly killing pigs and other small animals they want to use in their games. By the time they are ready to leave, the island is on fire and almost totally destroyed. The rather small scar of the airplane has turned into a giant, gaping wound that the boys have inflicted what was a paradise. This reinforces Golding's theme that man has an evil nature that must be controlled or man will destroy both himself and his environment.
What is the importance of the "scar"?
I'm assuming you mean the "long scar smashed into the jungle" described at the beginning of the novel? There are a couple of reasons Golding places such import on that word.
First, we, as readers have to understand exactly how everyone ended up on the island. The violence of their arrival is key, as it starts the process of their "decivilization". (Think how different the novel would be if a luxury liner pulled up the island on shore leave, and all the characters filed off). Second, Golding is letting us know that by scarring the jungle, humanity will now be in a battle against nature (one of the classic conflicts).