In tonal music chords have different status and function:
the tonic triad (I) is built on the first degree of the scale – a key’s most stable chord. Think of the tonic as having a strong "magnetic" pull for the IV and V chords.the subdominant triad (IV) is built on the fourth degree of the scale – a less stable chord that ultimately wants to resolve to the tonic. the dominant triad (V) is built on the fifth degree of the scale – a key’s most unstable chord. It has the strongest pull towards the tonic chord.
The structure of tonal music depends on the repeated use of a key’s three primary triads interacting between each other due to their relative attraction. Of the three chords, the tonic (I) is the stable chord. It often starts and ends a song. The subdominant is a less stable chord, and the dominant is most unstable. Harmonic progressions create a "push/pull" between chords that ultimately want to progress to the tonic. In the Beatles song, "In My Life" the song begins with music’s most stable chord, the tonic, which moves to a most unstable chord, the dominant, then back to the subdominant (IV) and then progressing eventually back to the tonic. This 4 bar passage illustrates the principle that Lennon & McCartney created tension and release through the skilled use of music’s primary chords (I, IV, and V). Learn more about music from Gene Kelly at kellymusicworks.com or follow @Kellymusicworks on Facebook.