Here is a REALLY great video that demonstrates functional harmony in a way anyone can understand. This is so well done.
For those of you who don't know what this term means here is an excerpt from Wikipedia that will help you understand:
In tonal music theory, a diatonic function (also chord area) is the specific, recognized role of each of the 7 notes and their chords in relation to the diatonic key. In this context, role
means the degree of tension produced by moving toward a note, chord or
scale other than the tonic, and how this musical tension would be eased
(resolved) towards the stability of returning to the tonic chord, note,
or scale (namely, function).
See the entire page here