One way to expand the harmonic possibilities of a chord progression beyond diatonic triads is to “borrow” chords from a parallel scale (a scale that has the same key center). The Borrowed Chords substitute (serve a similar harmonic function as) the chord in the original key.
If, for example, you are playing in the key of C Major, you might “borrow” the equivalent chords from the key of C Minor and vice versa.
Here are the diatonic chords in C major:
and the diatonic chords in C natural minor:
The “borrowable” chords always come in pairs that share the same number…
While it is theoretically possible to borrow any chord from any parallel key, only a small number are in common usage. Don’t sweat the details right now. For the moment, it is enough to understand the notion of borrowing.