Your question is very easy to answer, but it also has broader connotations.
In the book "The Lord of the Flies," a group of boys is stranded on an island with no adults. Two boys vie for the position of leader. One boy, Ralph, is good in his nature and represents civilized man. The other boy, Jack, is cruel, and his behavior regresses into savagery. Jack represents the animal nature of man.
Piggy is an innocent who wears glasses, has asthma, and is not a boy well-fit for the island. He is the weak link that is the victim. In a place where natural animal instinct rules, only the strongest can survive.
Piggy's glasses provide a power that is important to man. They have the ability to reflect the sun, causing heat to be directed onto leaves or twigs to start a fire. Jack knows that the owner of the glasses also is the owner of the power of fire. Therefore, he arranges to have them stolen from Piggy, who cannot see without them, except for blurred images. He needs the glasses to light the fire.