Watch the Scale Degrees online guitar lesson by Frank Vignola from Next Steps Jazz: 2-5-1 Progressions
Each degree of a scale is assigned a number. Each degree will be a different note depending on which key you play the scale in. This is a language that is used in music that helps to communicate which tones will be played at a given moment. The 2 - 5 - 1 chord progression is based on this numbering system.