In ch. 5, Ralph and Piggy prayed for a sign from the grown-up world. In ch. 6, they have been given one. Explain its significance. What is the sign?
(ie: the man that parachuted out of the plane that the boys thought was the beast)
I would also like to add that because the parachutist died as a result of war (a result of violence and cruelty), he truly does become a representation for what they fear. Although only Simon will realize it, "the beast" is actually the evil in each human. This darkness manifests as violence and savagery, as we see on the island. The boys give in to "the beast" and thus descend into savages who only care for huting and killing. Thus, the parachutist really does become the symbol of what they fear.
Also, Samneric's reaction to the form supports this idea of inner darkness. Jack picks up on their fear, and greatly exaggerates the danger for his own gain. So while the parachutist is mistaken for the beast, he also shows that the beast can come in form, from any of us. Unfortunately, Ralph will not realize this until two of his friends have been killed, & the other boys never realize it in the course of the story.
You're correct--the sign they receive is the man with the parachute that lands on top of the mountain. The significance of this event is that the man is dead, which could symbolize that the "grown-up world" they so miss is dead as well. Remember that the boys wound up on the island because they were escaping from a country that was being bombarded militarily. Figuratively speaking, the world they remember--a world of civilized behavior, of order, of stability--is dead. The experiment Golding is setting forth for us, then, is whether or not mankind can re-create civilization from scratch. It addresses the age-old question of heredity versus environment.
The other significance of this event is not realized for several more chapters. The boys--all except Simon, that is--mistake the boys for the "beastie," which leads to the climactic events that occur later in the novel.