.Secondary dominant chords are a series of six chords you can use to substitute for or augment the seven diatonic chords in major and minor keys.
There is a very simple formula you can use to calculate the secondary dominant chords to all chords in keys. (all chords except for the diminished chord) You simply count up a fifth from the root of each chord and you'll get the secondary dominant chord of each of the diatonic chords.
For example, we'll use the key of C, the one chord is C major, if you count up five steps above C you get a G note, then you make a dominant chord based on that root and you get a G7 chord, G7 is the secondary dominant of C. Here are the remaining secondary dominants in the key of C: Dm=A7, Em=B7, F=C7, G=D7, Am=E7. Those secondary dominant chords can be used in conjunction with the diatonic chords in the key of C, which are: C, Dm, Em, F, G, Am and Bdim to write chord progressions.
All six of the secondary dominant chords are commonly used in popular music and add a great deal of richness and harmonic interest to the seven diatonic chords.
A well known example of secondary dominant chords in a song is “Daydream” by the Lovin' Spoonful, the verse is: G - E7 – Am7 – D7 (the E7 is a secondary dominant of the Am) and the bridge is: C – A7 – G – E7 (the A7 is the secondary dominant of the Dm).