Chapter one of William Golding's Lord of the Flies is full of little bits of information which must be pieced together by the reader, and the scar to which you refer is one of them. Though it is one of the first things Golding describes for us, we are not sure what it is for several more pages.
Ralph is the first of the boys we meet, and he is walking near the scar when he meets another boy, also walking near the scar.
All round him the long scar smashed into the jungle was a bath of heat. He [Ralph] was clambering heavily among the creepers and broken trunks when a bird, a vision of red and yellow, ﬂashed upwards with a witch-like cry; and this cry was echoed by another.
“Hi!” it said. “Wait a minute!” The undergrowth at the side of the scar was shaken and a multitude of raindrops fell pattering.
Though the jungle foliage is obviously thick around them, some tree trunks are broken and heat is emanating from the scar, which is obviously a clearing smashed through the middle of the jungle.
The two boys, Ralph and Piggy, talk about what caused them to be here on this island, and it is the same incident which created the scar. They were on an airplane, there was a man with a megaphone, and there were other boys on the plane. Piggy wonders where the pilot is, and Ralph gives his opinion:
“He must have ﬂown off after he dropped us. He couldn’t land here. Not in a place with wheels.”
“We was attacked [said Piggy]! When we was coming down I looked through one of them windows. I saw the other part of the plane. There were ﬂames coming out of it.” He looked up and down the scar. “And this is what the cabin done.”
The fair boy [Ralph] reached out and touched the jagged end of a trunk.... “What happened to it?” he asked. “Where’s it got to now?”
“That storm dragged it out to sea. It wasn’t half dangerous with all them tree trunks falling.”
So, the scar is the result of the plane, or at least the cabin of the plane, crashing through the jungle on its way to its final resting place in the ocean. The plane had been hit by something in some kind of an attack (Piggy later says he heard about an atom bomb, which reveals the setting of the story as World War I); the pilot somehow managed to deposit the boys on the island but was unable to save himself or "the man with the megaphone."
In truth, the logistics of such an airplane crash are a little confusing, since all the boys managed to get out of the plane safely (and no one mentions parachutes), yet the plane was moving with enough velocity and force to create a huge gash in the trees and foliage of the jungle. (Ralph stands on his head in the scar when he realizes there are no adults on the island, so it is a substantial clearing.) This is a symbolic novel, however, and the most important things are that the boys are stranded here without any adults, they are here because of a war raging off the island (the boys were at boarding school and were being evacuated as the war encroached), and there is a great scar on the island--the very first symbol and foreshadowing of trouble ahead for these boys.