By doubles, we mean pairs of chords used together in an alternating manner.  For example, while a melody note is being sung in a chorus or hymn, the harmony could change quickly between a C and F chord (I-IV-I), a C and G chord (I-V-I), or what happens frequently in African Gospel music, a C and D minor chord (I-ii-I).  This technique is a basic harmonic idea used in Rock music as well. It’s one way to give movement and energy to a piece.