We kick off the novel with a bunch of bonfires, which are not just for dramatic effect – they set up a lot of the novel's themes. Bonfires are tied to the past and to history and give us a sense of the Egdon community. But we want to discuss two other major symbols here.
First, the bonfires at the start of the novel kind of act like lights on a stage.
Red suns and tufts of fire one by one began to arise, flecking the whole country round. They were the bonfires of other parishes and hamlets that were engaged in the same sot of commemoration. Some were distant, and stood in a dense atmosphere, so that bundles of pale strawlike beams radiated around them in the shape of a fan. Some were large and neat, glowing scarlet-red from the shade, like wounds in a black hide. (1.3.4)
Hardy sure knows how to rock out the descriptive imagery when he gets going. Props, Hardy, props.
So, given how many theatrical elements exist in this novel, it makes sense that we'd have a set of metaphorical stage lights turn on and light up the darkened heath, which is for all intents and purposes the stage of the novel. The novel has a lot of stylistic and structural ties to the theater, especially Greek dramas and tragedies. Greek plays were generally performed outside in amphitheaters, and this idea of an outdoor, natural stage setting runs throughout the novel and reinforces the idea of bonfire as stage lights.
But bonfires also act as another, perhaps more significant, kind of light in the novel: signal lights. Bonfires are the primary secret signal between Eustacia and Damon and we have multiple scenes in which they light a bonfire and have a clandestine, or secret, meeting. The description of this signal bonfire is very telling.
[T]his was the nearest of any, the moon of the whole shining throng. It lay in a direction precisely opposite to that of the little window in the vale below. [...] This quiet eye had attracted attention from time to time [...]. (1.3.117)
We can see here how bonfires act as eyes in a way – they help others to see what's around them and they call the attention of those at a distance.