Each chapter title reflects a large part of that chapter. In chapter 2, "Fire on the Mountain", the boys accidentally let their signal fire get out of control and they burn part of the mountain. They also note after the fire that one of the little boys is missing and he is never found.
In chapter 3, "Huts on the Beach", Ralph and his builders try unsuccessfully to build shelter for all of them. This is his first major display of inefficiency as a leader because the little boys would rather play than work. Ralph gets frustrated with them. While this is happening, Jack is sharpening his skills as a hunter and the rift between the boys grows.
In chapter 7, "Shadows and Tall Trees", the word "shadows" in the chapter title could refer to foreshadowing since there is a great deal of that in the chapter from Ralph's frustration with the inefficiency on the island and the growing differences among the two groups of boys to the hunt for the "beast" on the mountain (the dead parachutist).
In chapter 9, Simon is savagely killed by the boys themselves. They are all present which symbolizes, whether they took part in the actual killing or not, they are all to blame - they all saw what was happening. They all kill what could have helped them, namely Simon's news about the parachutist being just a person and that the problem on the island is the evil inside of each one of them.
Chapter 11, "Castle Rock" is about the fortress that Jack and his tribe have on this rocky outcropping that they have dubbed "Castle Rock" because it is here that Piggy is killed in another savage show of evil when Roger drops the boulder on Piggy's head. The chapter titles reflect the action that takes place within the chapters.