In the novel "Lord of the Flies" by William Golding, the author shows us the idea of fear of a vague, shapeless kind. Then he shows us how the primeval fear present deep in the psyche of all of us, is given credibilty and even a name. The boys had a mixture of emotions when they were wrecked on the island - some would have been excited at the beautiful freedom of their surroundings but mixed in with that would have been fear of the unknown and of the removal of adult care. One of the first to articulate this fear is the littlun with the birthmark - he first calls it a "beastie." Ralph tries to be reassuring, but the only interest Jack has in the younger ones is as slavish little gang followers.Fire on the mountain refers to how the boys found a paradise and turned it into a scorched hell of their own evil.
What you are talking about happens in Chapter 2. The littleun with the mulberry colored birthmark raises the question of the "beastie." He says it's a snake-like thing and he is afraid of it.
Ralph tells him that there is no such thing. But Jack starts talking about hunting it.
To me, the chapter is given this title because of what happens at the end. The boys set the fire, but they are so careless that it gets to be a huge fire and it ends up killing the boy with the birthmark.