What does harmonic progression mean and what are the rules?

Harmonic progression is the relationship from one chord to the next, and in the theory of harmony, it is the horizontal progression of chords. So, when assigning chords to a song, you should consider the horizontal progression in addition to the vertical relationship of the chords. There are many rules for this, such as the traditional harmonic approach, the popular harmonic approach, the pentatonic harmonic approach, and so on. It is necessary to take into account the tonal relationships, alternative chords, color chords, passing chords, transposition and dissonance, and also the harmonic progression, and so on.