What is this. I have been told that it is a good tool to know, but what is it? When i google it, I keep finding places to "purchase the secret". (ugh...) could someone explain it to me?

The Nashville Number System is an informal method of transcribing music by denoting the scale degree on which a chord is built. By writing chords as numbers, music may be transposed easily. As a simple system of transcription, it can be used with only a rudimentary background in music theory, and may not be as intimidating to learn for musicians unfamiliar with traditional notations. If a set of musicians has basic familiarity, improvisation and jam sessions can be quickly explained using numbers, and chord changes can be communicated mid-song by holding up the corresponding number of fingers. The system is flexible, and can be embellished to include more information (such as chord color or to denote a bass note in an inverted chord) as the user desires.