Chapter 8 talks about arrays, which are worth knowing about.  Skim quickly through Project 8 at the beginning of the chapter, and start reading more carefully the section titled "Arrays" that starts afterwards. Read carefully through the subsections "Declaring Arrays," "Using Arrays," ... and so on, beginning to skim again at the section on "Multidimensional Arrays." The remainder of the chapter is optional content on the limitations of the memory in the Arduino, and you can skim or read if this interests you.

Post questions or comments to the reading forum, as usual.