Answer by
lonelywolf (2117)
The numbers refer to chords built on scale degrees. In a major key, this is the 5 (or dominant) chord, followed by the 7 (or diminished) chord, followed by the 1 (or tonic) chord. In the key of C: G (G-B-D) followed by B diminished (B-D-F) followed by C (C-E-G).