6/4 chords, also known as second inversion triads, are chords where the fifth of the triad is in the bass, creating an interval structure of a sixth above the bass note and a fourth above the root. This unique positioning allows for specific voice leading techniques that help create smoother transitions between chords. In the context of harmonic progression, 6/4 chords often serve as passing chords or embellishments, connecting primary harmonies effectively and enhancing the overall harmonic texture.