This means that this notes corresponds to the strings played open without the fretting hand touching a single string. Well. what about if we color this notes with color markers. don't you think you can identify them more easily?
The major scale has a particular sound. You've heard this sound many times. You've probably sung it too. Have you ever heard anyone sing "Do, Re, Mi, Fa, So, La, Ti, Do?" That's it. That's the major scale. That's the alphabet.
Music has an alphabet too, but we call it a scale. Each note is like a letter. We put notes from the scale together to make chords (words). Then we put the chords (words) together to make phrases