This video shows how chords are contructed from the 7 notes of the Major scale.
Chords are built by initially stacking 3rds upon one another. Or, by taking the root note and
adding every other note until you have 3 notes for a basic chord or triad.
For example: The C major scale has the notes C - D - E F - G - A - B C
To create a chord with the C note as the root, we add every other note to get 3 notes... C E G.
That's your basic C Major chord.
You then do the same with the rest of the notes in the scale to end up with the following
chords: