The melodies of most Pop Ballads can be described as DIATONIC. What does this mean?

Most pop standard and jazz ballads are built from a single, introductory verse, usually around 16 bars in length, and they end on the dominant – the chorus or refrain, usually 16 or 32 bars long and in AABA form (though other forms, such as ABAC, are not uncommon)

Explanation:

Alternative Title: diatonic scale. Diatonic, in music, any stepwise arrangement of the seven “natural” pitches (scale degrees) forming an octave without altering the established pattern of a key or mode—in particular, the major and natural minor scales.