At the end of Chapter 5, Ralph, Piggy, and Simon lament about their separation from adults and wish they were present on the island with them. They discuss the many positive attributes adults' possess such as intelligence, courage, ingenuity, and magnanimity. Ralph says, "if only they could send us something grownup...a sign or something." (Golding 94)
At the beginning of Chapter 6, an aerial battle takes place over the island while the boys are sleeping. Golding describes the fast moving lights that "winked" in the sky from the battle. The images of moving light are caused by the bullets in the sky and are associated with shooting stars. This scene connects the boys' wishes to the bullets that resemble "shooting stars." Their wish comes true as a dead paratrooper falls to earth, which is described as "the sign from grownups." Golding compares the boys' admiration for adults with the reality of how adults behave. In the minds of the naive children, all adults are civil. The truth is that all humans possess savage qualities no matter their age. The paratrooper's decent is symbolic of Satan falling from the heavens. It is a tangible symbol of evil once it lands on the island and will become the catalyst of Simon's death.
You are referring to chapter 6. At the beginning of this chapter, a pilot ejected from his plane as there was a plane fight off in the distance. As the parachute comes down, the dead man comes down with it. Thus "a sign came down from the world of grown-ups."
What was this sign? Since there was a war and a dead man to prove it, the sign signifies all is not well in the world of adults. So, rather than setting a good example, adults set a bad one.
In fact, if we juxtapose what is going on with the boys on the island and compare it with what is going on in the world, we can say that there is depravity in the heart of man. No wonder the society of boys will unravel.
All of this foreshadows the greater trials of savagery the boys will face.