A chord progression is a sequence of chords, the harmonic path a piece follows. Musicians usually describe progressions with Roman numerals so they can be moved to any key: the 12-bar blues is built on I, IV and V (in C: C, F and G, usually as seventh chords), while the jazz ii–V–I in C is Dm7–G7–Cmaj7. On a lead sheet the progression appears as chord symbols above the melody, and jazz players refer to it as “the changes”; improvising “over the changes” means building new melodies that fit them.
Progressions create a feeling of motion and arrival: some chords sound like home, others like departure. Jazz lo-fi tends to use short progressions of two to four chords, borrowed from jazz and soul records, that loop for the whole track. Because nothing dramatic happens harmonically, the ear relaxes, which suits listening while studying or working.