Understanding what chords are, how they are created, and their function is important to playing well on any instrument. The number system derives from music theory concepts about how scales and chords relate to the key center and to each other. If you are new to music theory, don’t rush through these videos. I could easily write a post about the content of each of these videos (and I probably will eventually), but take your time and try and build an understanding of the concepts from one video to the next. Don’t RUSH.
If you don’t know much about chords or how they are created, then start with this video from Michael New.
This next video from Michael covers the major and minor keys.
The next is how to fit chords into a key (chord progressions).
And finally – the Roman Numeral System