This question is referencing an event that happened in chapter 8. Timothy and Phillip are working on a way to signal potential rescuers, build a shelter, and gather food and water. Timothy is doing most of the work, and Phillip likes to frequently remind Timothy and readers that he can't do much because he is blind.
Timothy has figured out a two-part signal. The first is a signal fire that will gain attention, and the second is to have something written in the sand with rocks so that the "mahn in d'sky" can see it. Phillip says he can't write anything because he can't see the rocks. Timothy's response is that he can see the rocks, but he doesn't know what to say. Phillip says to write the word "help." Phillip eventually figures out that Timothy can't write the word in the sand with the rocks because Timothy can't spell. Phillip then writes out the word in the sand with a stick and allows himself to feel superior over an old Black man that can't spell.
Chapter 9 contains a turning point in their relationship. Timothy will actually slap Phillip after Phillip makes another racially demeaning comment. The chapter ends with Phillip asking if they can be friends.
Chapter 10 begins, and Phillip and Timothy are having a cordial conversation. They are asking each other about their respective pasts. Timothy's description of his childhood is an eye opener for young Phillip because he realizes that Timothy's life experiences are far different. Timothy can't spell because he never went to school, and he was working as a fisherman already at age 10.