Here are two definitions of prosody, as the word applieds to songwriting.

Jason Blume:
To achieve prosody, the melody should sound as if it is happily married to the lyric it accompanies, and vice versa. A melody that makes the listener feel happy should accompany a lyric that has a positive, pleasant message. Likewise, a lyric that conveys sadness should be expressed melodically with notes, chords, and musical phrases that evoke a similar feeling of sadness. Minor chords typically contribute to a sad-sounding melody, while major chords can have the opposite effect.

Pat Pattison:
Prosody means that elements are working together for a common purpose, for example, when we line up words and notes -- matching stressed notes appropriately with stressed syllables. We could work with shape and weight in designing racing cars. We could create a relationship between rhythm and meaning by writing about galloping horses in a clippity-clop rhythm.