A borrowed chord is a chord temporarily taken from another scale or key that shares the same tonic as the key you are in. Using Fm or A♭ from C minor in a song in C major, for example, adds colors you cannot get from the basic diatonic chords alone.
Why borrow?
You can write a song with diatonic chords alone, but sometimes you want a touch of wistfulness, drama, or surprise. A borrowed chord lets you bring in a different sound for a short moment without changing the key itself.
An example of borrowing from the parallel key
C major and C minor are parallel keys, both felt around C. Putting an F minor from C minor into a song in C major gives you a shaded sound while keeping C as the axis. This is a different relationship from relative keys, so you can check the terms in relative key / parallel key.
How it differs from modulation
Modulation means moving the central key of the song itself. With borrowed chords you keep the sense of the basic key and bring in only some chords from outside. In practice the line can be blurry, but thinking of it as brief coloring is a good place to start.
How to try it in a DAW
Take a basic progression and swap just one chord for a chord from the parallel minor. If it clashes with the melody, move some of the notes to fit the borrowed chord. If the change feels too strong, use it for only a short time.
Choose by ear first
Judge by ear whether it fits the color the song wants, not by whether theory allows it. Borrowed chords are an option for when you want to add a little individuality to a standard progression.
