Passing chords are used as vehicles of movement between the foundational chords of a chord progression. They're not essential to the chord progression but the addition of them can add flavour to the progression. Such flavours as tension or suspense for example or even artistic flair.
Passing chords are often used in R&B and in this two part series we'll look at two effective passing chords and their function.
In the next tutorial we look at the half-diminished chord otherwise know as the m7♭5 chord. Check it out here.