A secondary dominant is a dominant chord that strongly leads to a chord other than the tonic of the home key, treating it as a temporary landing point. Placing a 7th chord a fifth above the chord you want to go to strengthens the sense of motion.
The basic idea
In C major, when you want to move to D, placing A7 creates a pull toward D. A7 contains notes from outside C major, but resolving to the D that follows makes it sound natural.
Its role in writing and performing
Use it when you want to add a slightly stronger sense of direction to standard diatonic chords. It works well just before a chorus, or anywhere you want to push a progression forward a notch.
How to try it in a DAW
In a basic progression, place the 7th chord a fifth above the next chord immediately before it and compare. If a melody note clashes with a note outside the key, use the chord briefly or adjust the notes.
How it differs from related terms
It applies the dominant-to-tonic relationship of a cadence temporarily to a chord in the middle of a progression. Its purpose differs from that of a borrowed chord.
When you want a little more pull into the next chord, a secondary dominant is a practical option.
