A Dominant chord is the 5th chord of the scale. For instance, G7 would be the dominant chord in the scale of C. A dominant 7 chord is root, 3rd, 5th and a flat 7th.
A Dominant chord creates tension or instability. Dominant 7 chords beg to be resolved by the tonic chord. Meaning playing a G7 while in the key of C, makes the C chord feel like home. It doesn’t have to be a seventh chord, but the resolution feels much stronger with the seventh.
- E7 resolves to A
- F#7 resolves to B
- G7 resolves to C
- A7 resolves to D
- B7 resolves to E
- C7 resolves to F
- D7 resolves to G
A Secondary Dominant Chord is five steps away from V chord. So if your V chord is G7, D7 would be the secondary dominant.