Music is made up of three basic elements - rhythm, melody and harmony. You have already learned the basics of rhythm and melody in this book. Harmony means other notes which accompany the melody. A good example of harmony would be a guitar or keyboard playing chords.
Even if you mainly plan to play the flute, it is a good idea to learn a bit about chords and how they work, so you can understand what other musicians are talking about when they accompany you and you can also understand the sounds the instruments are making and why the musicians choose those sounds.
The best way to learn more about harmony is to learn how to play an instrument such as guitar, piano or electronic keyboard. As well as learning chords, this will help you to hear and understand melodies better and give you the tools you need to write your own songs.