In the key of C major, the primary dominant is G7

Secondary dominants are dominant chords that resolve to all the other chords in the key of C major.

These ones: Dm, Em, F, G and Am

Nobody has ever resolved onto a minor seven flat five chord without sustaining physical injury, so don’t even try to do that.

The first example could be a harmonisation for Tiptoe Through The Tulips. That’s not what the actual chords are, but it sounds fine, and it gives you a handy way to remember the first few secondary dominants in ascending order.

The second example is all the secondary dominants resolving to their diatonic temporary homes.

It’s a neat way of getting from C to Am in 6 bars.

Useful, eh?