If you play all the notes from the open, A on the 2nd string to the A one octave higher on the 1st string, they form the A natural minor scale. A scale is a combination of notes which is traditionally used to create melodies.
wScales start and finish with notes which share the same letter name but are an octave apart. The following example demonstrates the A natural minor scale. Memorize the sound of the pattern and then try creating your own melodies from it. You will learn more about scales as the book progresses.