A Chord Progression is a sequence of chords that achieve a larger-scale harmonic purpose than can be achieved by a single chord alone. These purposes include establishing a key center and tonality, building harmonic tension, releasing harmonic tension, creating a sense of forward motion and anticipation, changing the mood, introducing tonal ambiguity, and changing key center.
Studying Chord Progressions is essential to understanding how an enormous amount of music works. Doing so expands your musical literacy, deepens your theoretical understanding, heightens your aural awareness, and develops practical performance skills like sight reading, playing from memory, playing by ear, improvisation, and transposition.
As your network of harmonic understanding grows, you will discover that many pieces that seem very complex on the surface are based on some very simple harmonic ideas, revealing both the simplicity and creative genius behind the music you have come to love. Enjoy the process!