What is prosody?
The match between the natural stress of spoken words and the stresses of the melody. Good prosody makes a lyric feel inevitable; bad prosody makes a singer fight the line even when every syllable technically fits.
The usual culprit is an important word landing on a weak beat. Reordering a phrase often fixes it without changing a single word choice.
The number of syllables in a lyric line, which determines whether it fits the melodic phrase. Counting is the quickest way to spot a line that reads well on paper but cannot be sung in the space available.
The most memorable part of a song — often the title line, sung at the point the listener is most likely to remember it. Hooks are usually short, repeated, and built from plain, concrete language.