The beast signifies different things to different people: both characters in the book and readers. At its most basic level, the beast is associated with the dead airman who parachutes over the island in the Chapter Beast from the Air. Sam and Eric make this association. Symboloically, therefore, the war that wages outside the island, which ahd also caused the boys to be stranded initially, can be viewed as the beast.
The beast also symbolises the boys' fear and fear of the unknown. In Beast from the Water, the multiplying theories about the beast indicate the mass panic and terror taking the group. The various descriptions of the beast also connect it to the jungle which may symbolise its connection with the boys' increasing savagery and barbarism.
Simon may come closest to describing the beast as "us": an inherent and internal 'evil' or savagery which all humanity carries around and which becomes explicit in characters like Jack and Roger. It is highly ironic and appropriate that Simon identifies the 'true' nature of the beast and is killed as the beast.
To a religious reader, the concept of the beast is readily associated with both Satan and the concept of original sin. In this interpretation, the island becomes a second garden of eden, the boys' descent into savagery a second Fall.
To characters like Jack, the beast becomes a method of control through fear and intimidation. In many ways, Jack is the embodiment of the beast.
The beast most likely symbolizes the savagery that exists within all humans. The beast is real only in the minds of the boys. As their morality declines, the beast becomes more and more real, until they treat it almost as a god. Simon was the only boy to recognize the beast for what it really was.