The circle of fifths is the way the 12 musical notes of the chromatic scale are arranged following a sequence of perfect fifths (7 semitones). It helps visualizing relationships between notes and better understanding diatonic harmony.
One of the fundamental theoretical elements to understand music is the harmonization of the major scale. Harmonizing scale is building chords with notes. This lesson explains how to create triads and seventh chords from each note of the major scale.