In "Lord of the Flies" fire is representative of three things: hope, power, and destruction.
In the beginning of the novel, fire represents hope. The boys agree to build and maintain a fire as a signaling device to passing ships. The fire is their mechanism to achieve rescue; therefore, the fire is a symbol of hope. It is giving the boys hope to go home and links their efforts to a return to civilization.
Fire is also symbolic of power in the novel. Whoever has the ability to make fire is the more powerful leader or "tribe." Perhaps Piggy's glasses represent this power element better than the fire, but fire is the end result of the glasses. The glasses are simply a tool to start the fire. By attaining the glasses later in the novel, Jack becomes the more powerful leader, because he has the ability to make fire.
The third thing that fire represents is destruction. It is the signal fire that winds up killing the first boy. Jack also uses fire to hunt down Ralph, which is ironic because it's that fire that ultimately signals a rescue ship.