The Lord of the Flies, or the beast, is a metaphor of the natural chaos that exists within human nature. The transition from boyhood into adulthood includes the conversion of that chaos into a desire for order, in most cases, but the lack of any adult supervision in the wake of the abandonment of the boys means that the chaos goes largely unchecked.
Several other metaphors are at work in William Golding's classic novel. The conch shell, for example, is a metaphor for the law of the adult world that the boys have left behind. Piggy makes it his mission to protect the conch because the notion of order is soothing to him. However, after Roger demolishes the conch, the boys no longer have a sign of authority in place, and anarchy reigns on the island.
The island itself is a metaphor for the isolation that the boys have from the rest of their society. Without any connection to outside society, the boys have to solve their problems themselves. The small island becomes an ideal crucible for the weaknesses in human nature.
The glasses are another metaphor of the voice of logic and reason among the survivors. Piggy keeps an even tighter grasp on the glasses than he does the conch, using them to solve the boys' problems, most notably the kindling of the fire, which gives the boys their best hope of a rescue.