Have you ever noticed how certain chords just fit well together? Have you ever noticed that when playing your favorite songs, you often times play the same chords and sometimes even the same exact chord progressions? The reason that you are noticing this is that you're discovering a "code" that underlies all music. The NNS helps us to decipher this code so that we're no longer left guessing which chords work well together.
If we harmonize a "3rd" and "5th" off of each one of our seven major scale notes, we get three different triads (as discussed in the previous lesson). In a major key, these chords are as follows:
If you were to analyze a hundred classic songs, you would find that about 99% of the chords would follow the above structures. It's not to say that artists can't be creative and use other chords, but the chords that sound most natural are the ones that are "hard-coded" into and dictated by our major scale.