Watch the Harmonic Concepts online guitar lesson by Matthieu Brandt from Songwriting on Guitar
Aside from the I, IV and V chord in a major key, there are three other minor chords we can use.
Chords on the second, third and sixth degree are all minor and all these chords can function as replacement chords for the three most important ones. The chord on the seventh degree can be ignored, because it's unstable. It's a diminished chord and is never used in a song in a major key. The three minor chords from a major key give you more chord options and extra tools (ascending and descending chord progressions) to shape your songs. The minor chords all have a 'relative' relationship with the chord they can replace. Their root can be found 3 half tones (or frets) down from the relative major.
They also open up the possibillity to withhold one of the building block chords I, IV or V. Just use the replacement relative minor and when the I, IV or V chord does come along it's impact will be bigger.