A borrowed chord is one taken from a key that shares the same tonic but has a different mode, most often a minor chord used in a major key. Theorists call the technique modal mixture or modal interchange. In C major the parallel key is C minor, and the usual borrowings are F minor (F–A♭–C) instead of F major, A♭ major, B♭ major and the half-diminished Dm7♭5. The common thread is the lowered sixth note, A♭, and sometimes the lowered seventh, B♭.
The effect is a sudden shadow inside a bright key. Jazz composers used it deliberately: Cole Porter’s “All of You” includes an F minor chord and a half-diminished D chord within a major-key tune. In jazz lo-fi a loop that moves from Fmaj7 to Fm7 and back to Cmaj7 is a classic way to make a simple progression sound nostalgic, as if the music remembered something slightly sad.