The mountain represents the struggle that is ahead of the boys. They must struggle to survive the conditions. They must struggle to survive among each other. The mountain is a natural force that can be climbed, if one is willing to put forth the effort. Civilization can be maintained if people are willing to put forth the effort.
it is significant because its where the beast is and where the fire is. the boys have to overcome their fears to be rescued because they have to light the fire.