The dead parachutist can represent quite a number of things. Firstly, he represents contact with the outside world, something which Ralph has been longing for. It is, however, tragic and unfortunate that he is dead; he is of no use. This further suggests that the war being fought away from the island serves no purpose--no one will be rescued from the evil inherent in man. Just as the boys cannot be rescued by the parachutist, so can the war not save mankind.
Secondly, the parachutist represents man's self-destruction. Since he is from the world of civilization, the expectation would be that he represents what is good about it, for the word "civilized" has positive connotations. He, however, represents everything that is bad about civilization. He is dead and is a victim of man's seemingly innate desire to destroy. There is a war going on and he becomes a symbol of its destructive nature.
In the third place, the parachutist, once he has been seen by Sam and Eric, comes to represent the physical reality of the boys' fear of the unknown, something they sense is pervasive and evil--which they have named the beast. The movement of the parachutist's body, caused by the wind, so terrifies the two boys that they run as fast as they can to inform the others that they have actually seen the beast.
Furthermore, Golding's indirect representation of "the beast" actually being a man ties in with Simon and Piggy's sentiments that the beast is just us. It is not something external but an inalienable part of our inner being. The beast is a man and, therefore, the beast is every man. This much is proven by the boys' desire for savagery and their lust for blood. Even the most decent and civilized of the boys, Ralph and Piggy, sense this urge and respond to it. Ralph does so when he throws his stick at a pig and hurts it and when he later participates in the hunting game and he feels the deep urge to hurt Roger, who acted as a pig being hunted. The desire also finds expression when they inadvertently participate in Simon's murder, thinking that they are killing the beast.
In addition, the parachutist also represents man's fall from grace. It could also, in terms of Christian belief, represent the angel Lucifer's banishment from heaven. The parachutist has come from the sky, which symbolically represents heaven with the obvious connotation of good. He is dead and later rots, which further represents the inner cancer which is rotting man from the inside.
On another level, the fact that the parachutist is still trapped by the lines that hold him to the rocks alludes to the reality that man is himself trapped. This point is most aptly illustrated: only after Simon has cut the body loose from its trappings can it be carried away. The implication could be that man has to be released from his malice to be fully free.
The dead parachutist is symbolic of the "beast" the boys fear. The boys fear a beast that is mythical in nature, yet the discovery of the corpse puts their fears in a physical form.
Previously the beast was not a fleshed out figure to the boys. Whatever fears each boy might have had created their own personal idea of what the beast was.
Because the parachutist obviously experienced a violent and tragic death it also is symbolic of what the boys fear most. The rotting corpse is symbolic of the rotting away of civility and order on the island.
For short it means the fall of mankind.