This quote appears in chapter 4 of William Golding's book Lord of the Flies. The quote is focused on Roger and his desires to throw stones at Henry.
Roger is an interesting character, because he so clearly shows how a person can be controlled by operant conditioning. Roger has been conditioned to expect some kind of punishment for mean (savage) behavior. He wants to throw stones at Henry, but civilization has conditioned a sense of fear in Roger. We see in this moment a Roger that is beginning to test those civil boundaries. He is beginning to let his savage side push against the bars holding that side back. This explains why he does throw the rocks at Henry in the first place, and it explains why he waits until he can't be seen by Ralph and does not actually hit Henry. His desires are clashing with his fears.
Roger has always had civilization and savagery warring within him, but he has always been surrounded by civilization's teachers, police, rules, and so on. Now that he is on the island, those controls have been removed, and Golding is showing readers that those emotions are clashing with each other. Roger will eventually fully embrace his savage nature, join Jack's crew, and not miss Piggy with a much bigger rock. Roger has indeed come a long way from being the boy ruled by civilization who couldn't bring himself to hit a kid with a rock: he becomes a savage who kills a kid with a rock. When savagery and civilization clash, savagery wins out if societal controls are not in place.