The Chords and Scales of G Major
Chords and scales are related. Almost all chords are derived from scales. A "G" major scale is:
G - A - B - C - D - E - F# - G
To get a G major chord from a G major scale you start at G and skip every other tone.
This produces a G Major triad chord:
G - B - D
If you start on "A" the second note of a G major scale and skip every other note, you get an A minor chord:
A - C - E
Starting on each G major scale tone, a family of eight "diatonic" chords are produced. These chords are either major, minor or dominant.
Likewise, if you play eight notes of a G major scale starting on every tone, (G to G, A to A, B to B, etc.) eight matching scales are produced. The eight chords and scales of a major scale are the definition of a key or tonal center. The G in the eighth position of a G major scale is a G one octave higher and is fretted in the standard G major chord position at the fifth fret.
Here is the tablature of the exercise shown in the video. More detailed information on chord and scale relationships can be found in the Mel Bay book "Straight Ahead Jazz For Banjo."