What is the "sign" from the adult world in "Lord of the Flies" and its literal and figurative meanings?

Literally, the sign that comes from the world of grownups is a dead paratrooper who falls from the sky and gets tangled in the trees at the top of the island.

The twins Eric and Sam see it and are frightened. At this point, it takes on figurative meanings. To them it is the dreaded "beast." They describe it as follows:

"It was furry. There was something moving behind its head—wings. The beast moved too—" "That was awful. It kind of sat up—" "The fire was bright—" "We'd just made it up—" "—more sticks on—" "There were eyes—" "Teeth—" "Claws—"

This fantastic notion of the dead paratrooper as a beast becomes the "truth" the boys now accept. The paratrooper morphs into the symbol of what is threatening, feared, and to be destroyed. The boys project onto it their own fears and make it larger than life, a monster rather than a corpse. Primitive emotions swamp reason and lead to bad outcomes when Simon is mistaken for the beast.

At the end of Chapter 5, Ralph and Piggy lament about their situation and comment that they wish "grownups" were around because grownups "know things." Ralph says, "If only they could send us something grownup...a sign or something." (Golding 94) At the beginning of Chapter 6, an air battle takes place over the island while the boys are sleeping. During the battle, a paratrooper gets shot out of the sky and lifelessly floats to earth. The trooper's parachute cords get tangled in the canopy of the forest, and his dead body moves with the wind on top of the mountain. Golding utilizes dramatic irony by answering Ralph's request for a sign from the grownups with the corpse of a paratrooper. The paratroopers descend from the sky symbolizes Lucifer's fall from heaven. Later on in the novel, the dead paratrooper is mistaken for the "beast." The "beast" represents the inherent evil present in every human which correlates with Lucifer's nature. Literally, the "sign" from the grownups is the dead paratrooper, and figuratively the paratrooper symbolizes the existence of evil on the island and alludes to Lucifer's fall from heaven. 

What sign do the boys receive from the adult world in Lord of the Flies?

At the end of Chapter 5, the boys lament about their situation and wish for a sign from the world of grown-ups. Ironically, the sign from the world of grown-ups is a dead paratrooper. At the beginning of Chapter 6, while the boys are sleeping, an aerial battle takes place above the island. In the midst of the battle, a paratrooper is shot down out of the sky and drifts to earth. The boys are unaware of the overhead battle that took place throughout the night and do not know that a dead paratrooper has landed on the island. Later on in the novel, Samneric spot the dead paratrooper's body swaying back and forth on the top of the mountain. Samneric mistake the paratrooper for the beast and the boys form a hunting party to track the beast. Simon is the only character to view the paratrooper and correctly identify it as a dead human body. Unfortunately, Simon is violently assaulted and dies on the beach before telling the boys the news the beast is actually a dead paratrooper.

What does "a sign" symbolize in Lord of the Flies?

In Chapter 5, as Ralph, Piggy and Simon sense the decay of order in the group, they momentarily wish for the comforts of the familiar adults that, until being stranded, had always provided guidance and discipline. Ralph abstractly wishes that the adults could send "a sign", meaning a message. Ralph doesn't articulate this thought, but it's pretty clear that he wishes that the adults could provide that same guidance and discipline, a sort of reaffirming of the right way to act and think, as well as assurance that the boys had not been forgotten and would be rescued if only they could keep their heads and wits.

The "sign" that comes arrives a few hours later, when all the boys are asleep and unable to see the contrails and explosions of what is evidently a fight between aircraft several miles above ground. The battle appears to culminate in the destruction of one aircraft and the parachuted body of its pilot lands on the island, and is later mistaken for the Beast. The figurative message this sends is that the adult world currently has its own problems, the war is ongoing, and it is the most sure source of death that the boys could possibly find. It is another reminder that the "Beast" is inside us, as well as a reminder to the boys (though they don't know it yet) that the "infinite cynicism" of adult life is what awaits them should they escape. 

In Lord of the Flies, what is the "sign" that is given?

The "sign" that is given is unnoticed by the boys. While they are sleeping, a dogfight occurs above them between planes fighting the war the boys were trying to escape. One of the planes is hit and the pilot ejects and parachutes down to the island. The parachutist dies but they boys are unaware of the entire battle. Later, the dead parachutist is mistaken for "the beast". This reconfirms Golding's theme that the beast is human, or at least part of humanity and the evil the boys exhibit is part of them.