During the course of this book you have learnt to play songs using both chords and single notes. Any line played in single notes is called a melody. Any other accompanying notes such as chords are called harmony. There are many ways in which melody and harmony are used in music.
One of the most common combinations is to have one instrument play the melody (e.g. harmonica) and another instrument play the harmony (e.g. guitar). When you play a song using chords on the harmonica, usually you are playing a combination of melody and harmony at the same time.