The fire that was made using Piggy's glasses represent technology. This "technology" of theirs can be their ticket out of the island and a way to get back home but it can also be threatening if not used carefully which at some point they didn't.
The rules of the fire was that it would be on every day just in case there was someone near and they could be rescued. The boys in charge of the fire were Jack's hunting group.
The signal fire is so important because it represents hope to the boys and proof that there's still some sort of civilization in them. However, whenever they allow themselves to turn it off, that's usually when they allow their savagery to come in.