How does one describe what important items in "Lord of the Flies" represent allegorically?Please include the definition of Golding's novel as...

An allegory is a story with more than one level of meaning--a literal level and one or more symbolic levels.  Seen as an allegory, William Golding's "Lord of the Flies" has the island represent a type of Garden of Eden in which innocent man arrives in the form of the boys.  Ralph, the golden haired born leader and Piggy, the intellectual and rational counterpart lead the other boys until Jack and the other hunters, the savage side of man vie for power with them.  Roger, whose evil nature has only been curbed by conditioning in his society--at first he throws rocks only near Henry--sheds all trappings of this civilization to later become consummate evil.  The Lord of the Flies, the beast, is the devil, who talks to the spiritual boy, the intuitive Simon, and tells him, "You knew, didn't you?" meaning that Simon sensed that the "beast" is the evil and violence inherent in man. 

On a symbolic level, it is the forces of nature that bring the beast to the boys spiritually.  Like the rotting pig's head, the parachutist is also rotting, symbolizing the deterioration of the spiritual, wholesome part of man.  The boys perceive him as a large ape, thus suggesting their regression to a more primitive, violent state.

The conch is like a horn that calls the boys to meetings, so it symbolizes order/society's rules.  The fire is the signal to the boys' society, their hope of rescue; for Ralph and the others, the fire is of paramount importance.  And, for Jack and the other savages, it also represents power.  When he and his hunters steal the fire, an allusion is made to the mythological story of Prometheus who stole fire from Zeus and gave it to the mortals, thus empowering them.

Other important allegorical symbols are the anonymous masks behind which Jack and the others hide in order to act out their cruelty with impunity and Piggy's glasses, also an empowering instrument, which enable the boys to light the fire.  For Piggy, of course, they allow him to see and, thus, be in control of himself.  When his glasses are stolen, the symbolic meaning is that the rational side of the boys has been defeated

Even the ship that rescues the boys is not without an inherent evil, for it is a battleship and the man a naval officer, both of whom suggest war and killing.