When you say, I - IV - V, what does it all mean? It means functionality and feelings! Well, it turns out that each of the big chords in a key (like I, IV & V) can cause some pretty predictable feelings. Those feelings are really what help us define the relationships between chords in a diatonic scale. It's pretty cool to realize that all the math of music theory lines up so well with our emotional responses to chord progressions. Listen to Bonnie Hayes explain basic harmonic functions present in all western music, in terms of function AND feelings. Pretty cool stuff.