6/4 chords, also known as second inversion chords, are triads where the fifth of the chord is the lowest note, creating a distinctive sound. This inversion is labeled '6/4' because it is built on the interval of a sixth above the bass note and a fourth above it, often used to provide smooth voice leading and create passing motion in harmony. These chords serve various functions in progression, often acting as a transition to more stable root position chords.