What Is a Chord Progression?
Simply put, a chord progression is a movement of chords from one point to another. If you've ever heard a blues song, you've heard a progression of chords. All the pop music from the last 100 years is loaded with chord progressions. If you play guitar or piano, you know all about chord progressions. The trick now is to figure out what they are, why you need to know them, and, more important, how this information is going to help you.
When you studied how to make chords, you looked at the chords a few ways. First, you stacked diatonic notes from the scales and ended up with seven different chords. You also dissected the intervallic properties of every triad and seventh chord in existence. This is one way of looking at chords. But there is another angle. When you talk about chords as vertical stacks of notes, you essentially are adopting a philosophy that chords are objects.
Chords and melodies are tied together very tightly. When you play a melody, you can almost imagine what harmony is present with that melody. As a result, it's possible to think melodically and harmonically at the same time. Beyond the theoretical underpinnings of what melody notes fit with which chords, when you listen to a melody, that melody has a way of telling you what chord it wants to have accompany it—all you have to do is listen.
The Chicken or the Egg?
The concept of vertical stacks works pretty well in studying a single chord. Looking back through the development of music shows that chords, although they are vertical stacks of notes, are closer to being vertical collisions of voices. What does this mean? Well, imagine that you are not playing guitar or piano; you are in a choir. There are soprano, alto, tenor, and bass voices. At the simplest level, there is one singer per part. Is a person in the bass section singing one note at a time, singing chords? No, she's singing a line—a melody, to be more specific. Since one voice can't make a chord, you have to look at the net result of what the choir is singing. There are four melodies going on at once. Each part is different. Now, if you freeze any single slice of vertical time, you could look at all the notes that are sung on the first beat of the first bar and come up with a chord. That would make sense because music should sound rich and consonant, and chords and harmony allow this. Now ask yourself which came first: the individual lines of music or the chords, and the voices simply fleshed out the chords as they went along?
The answer is complicated. It's hard to say for sure because most of the composers are dead. However, throughout the development of music, especially classical music, lines ruled and chords were afterthoughts.
Put simply: Composers wrote lines of melodies that summed together as chords when musicians looked up at them (vertical thinking). Since music theory has a wonderful ability to look back at composed music, it's easy to forget that lines were dominant.
How about Now?
Is music any different now? Depends who you ask. Do singer/songwriters write in lines? Sure, they sing a melody line, but do the chords they play on their guitar or piano come from that same thinking (linear)? Nowadays, especially in pop music, chords and chord progressions are units that have little to do with the old model. That's not to say that they can't, and that popular music has no voice leading in it, but the largest amount of popular music is conceived with chords as blocks of information, and melodies are layered on top of the chords. Now, all musicians—old school and new school—can learn from each other. Look at history and see what happened.