When the boys first light the fire, it is intended as a signal. Ralph, representing the more rational and rule-bound faction of the boys, sets up shifts and tries to organize the boys so that there will be a fire all the time so that any passing plane or ship will be able to see them and they can be rescued.
But the boys struggle to maintain this, in some ways suggesting perhaps their struggle to maintain the trappings of civilization. This is emphasized when they see a plane only to then find out that the fire has gone out.
As the story progresses, the use of fire changes to be something the boys basically fight over, the power to light the fire resting in Piggy's "specs." This too may symbolize a shift in power once the hunters take the specs and the power of fire.
Lastly, the great fire that burns much of the island as the hunters pursue Jack also leads to their rescue, a twist that is perhaps ironic given that in this moment the last elements of civilization are stripped away and the boys are hunting one of their own; only then does fire brings a return of adults and the rules and civilization they represent.