Answer
It just means note five from the scale degrees in ascending order: 1.2.3.4.5.6.7.8.or..tonic, supertonic, mediant, subdominant, dominant, submediant, leading note, tonic(octave).so in c it would be the chords : c dm em f g am bdim c.and the scale notes c d e f g a b cthe dominant 7th chord is the dominant chord with the 7th note added so again in c.. count to the fifth note g and add the 7th note away from it which is f.so this will give you the chord g7
