After the boys have crash landed on the desert island, they need to set about exploring their new surroundings to get the lie of the land. So Ralph forms a group to go on an expedition. Piggy begs Ralph to let him tag along. And why not? Piggy's by far and away the smartest boy on the island. His intelligence and capacity for logical reasoning could prove very useful on the expedition.
But Ralph tells a dejected Piggy that he won't be needed; this just isn't the right kind of job for him. As we might expect, Jack's even more blunt, telling Piggy right out that he isn't wanted. The thinking behind the decision to leave out Piggy is that he's likely to slow the other boys down on account of his excess weight. Piggy's deeply disappointed at being left behind to collect the names of the other boys, but there's really not much he can do about it.
There is ominous significance in this episode. Piggy, the voice of reason on the island, is simply not respected. It's only the first chapter in the story, and yet already it's clear that the values of reason and civilization that Piggy represents have no place in this savage, brutal environment.