The quote means that the boys are offering up the pig’s head as an offering to the forest.
As the boys begin to hunt, they become more and more heathenish. When they kill the pig, they decide they need to make an offering with it.
“But we’ll leave part of the kill for . . . ” (ch 8)
He does not finish his sentence, because he cannot articulate exactly what he is doing. He just has a primitive impulse to leave part of the pig as an offering.
In the silence, and standing over the dry blood, they looked suddenly furtive.
Jack spoke loudly.
“This head is for the beast. It’s a gift.” (ch 8)
When the forest replies with “silence” the boys take that as acceptance. They have made their kill, and they have made their offering.
The hunters have tasted blood now. They feel the immense power that comes from successfully taking another life. The power makes them respond in a way that seems natural to them. They offer up the pig’s head to the beast, an unknown power in the forest.