The setting of William Golding's Lord of the Flies is a deserted island; the only inhabitants are a group of proper English schoolboys without any adult supervision. Golding said he wrote this novel in "an attempt to trace the defects of society back to the defects of human nature," and this setting and these characters offer a blank slate on which he can display both society and human nature.
What happens on this island is a microcosm (small world or small picture) of what is happening in the adult world off the island. The novel is set during World War II, so the same kind of battle that is raging on the island is being fought in in the rest of the world.
After Jack begins to paint his face, everything on the island changes dramatically. He sees his reflection in the water:
He looked in astonishment, no longer at himself but at an awesome stranger. He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness.
Without the restraints of law (adult authority), shame, or conscience, Jack pursues his own base, selfish desires which results in several murders. Worse things would certainly have happened if the force of law (the naval officer) had not intervened.
Off the island, adults who have allowed their basest, selfish desires to control them (which is what happened to Jack) are trying to usurp power from everyone else. Others are are fighting to maintain order and discipline (like Piggy and Ralph), while some are fighting for man's humanity (like Simon).
The conflagration and battle on the island would have eventually killed everyone if the naval officer and his ship had not come to the boys' rescue. The real would have eventually destroyed the world (both in a literal and figurative sense) if people had not banded together to fight and stop it.