Trope: a literary figure of speech. Tropes are often used to refer to familiar literary devices such as metaphors and other types of figurative language.
This is a subtle topic and one I can best explain by using the lazy man’s approach of pointing you in the direction of someone else’s definition. So, I give you this link and hope you’ll forgive my indolence on this occasion: http://grammar.about.com/od/tz/g/tropeterm.htm
It is worth viewing this site, though. And the examples given lower down the page say it much better than I could.
Pic: Grain crop ready for harvesting.