The boys had been sent away from their families at the onset of a nuclear exchange. Similar to the way that so many children were sent away from London during WWII, the idea was to get them to safety. But their plane crashed and none of the adults were in the portion of the plane that survived so the boys found themselves alone on the island.
Piggy is described as very fat, with bad eyesight that requires him to wear glasses, and physically awkward. He doesn't walk very confidently, cannot swim, and is turned away from the party that decides to climb the mountain to begin an exploration of the island and to confirm that it is an island.
Ralph, on the other hand, is described as being broad shouldered but still young. He has an imposing physique for his age and is good looking, making it easier for the boys to look up to him. He is athletic and strong, a likely candidate for leadership among a group of frightened boys.