Secondary dominants are chords in music theory that function as temporary dominants to a chord that is not the tonic, or home, chord.
They are called "secondary" because they are not part of the original key or scale of the piece, but are instead borrowed from a different key or scale.
A secondary dominant is typically a dominant chord (V or V7) that is applied to a chord other than the tonic (I) chord.
For example, in the key of C major, the dominant chord is G7 (which includes the notes G, B, D, and F).
If the chord progression moves to the iii chord (E minor), a secondary dominant can be used to lead into it. In this case, the V7 chord of E minor is B7 (which includes the notes B, D#, F#, and A).
By using the B7 chord, it creates a temporary "tonicization" of the E minor chord, leading to a stronger resolution back to the original key of C major.
Secondary dominants are often used to add variety and interest to a chord progression, and to create more tension and resolution in the music. They are commonly found in many styles of music, including classical, jazz, and pop.