Secondary dominants (V/V, V/vi, etc.)

Concept

A secondary dominant is 'the V of' another chord in the key. In C, the V of V is D7 (the dominant of G, the actual V). Insert D7 before G: C - D7 - G - C. The D7 isn't in C major (it has F#) but it pulls hard into G — making the V→I cadence feel inevitable.