Answer:
Dominant and sub-dominant refers to notes of a scale. The dominant is the fifth note (represented with a roman numeral, V) of a scale while the sub-dominant is the fourth (IV) note of that scale. For example, in scale of C major, the dominant is G and the sub-dominant is F.
The terms dominant ans sub-dominant can also refer to chords, scales or keys. A dominant chord is one that is built on a dominant note. Musically, the dominant chord is considered to be unstable and must be resolved. Therefore, a dominant chord can be used to build tension in a chord progression.
Dominant keys refer to the relationship between notes. For instance, key of G is the dominant key relative to C. Music that changes key often shifts between a tonic and its dominant.