Secondary Dominant refers to the use of dominant chords of any diatonic chord. Diatonic chord in simple and practical terms mean ‘in the key of’. So the diatonic chords in the key of C are:
C Dm Em F G Am Bdim
If we use the dominant chord for any of these chords, which happens to be a non-diatonic chord in the key of C, we can label that as a secondary dominant. To know what is meant by the dominant chord, please read this (scale degree). For example, the dominant chord in the key of G is D. We can see that G is a diatonic chord in the key of C. However, D is not and in the key of C, we only have a Dm. The use of D in a chord progression written in the key of C will be labelled as a secondary dominant. The best way to use the secondary dominant is to play that chord, then follow it up with the tonic chord related to that dominant and then move to other diatonic chords. The following chord progression is an example where the progression is written in the key of C and D is the secondary dominant.