Learning harmony can be daunting for many musicians but I believe that if you understand harmony well it simplifies most musical styles so that you can gain a greater understanding.
It’s also invaluable for Jazz or commercial musicians.
You can download the PDF introduction to Jazz hamony below. Understanding this single sheet is probably more important than any of the more advanced lessons that you may come accross. It’s easy to skim over this thinking that you know it, but beware, you must understand it!
This sheet deals with the naturally occuring diatonic (major scale) chords (3 note chords) as well as the 5 qualities of chords that we encounter in Jazz playing (based on 4 note chords).
Thinking in numerals (or Roman numerals as here) is important because you can change key easily with refernce to only a single chord sequence (II-V-1 as opposed to Dm7-G7-Cmaj7)