Trope: The word trope can refer to any type of figure of speech, theme, image, character, or plot element that is used many times. Any kind of literary device or any specific example can be a trope.
The Enemies to Lovers trope = Enemies to reluctant allies to friends to lovers. Enemies to lovers trope is when two characters start off as enemies and, over the course of a book or series, end up in a romantic relationship. These 'enemies' have to overcome their differences or misconceptions about each other, and in the process, they fall in love.