A trope is a word used in a nonliteral sense to create a powerful image. If you say, "Chicago's worker bees buzz around the streets," you're using a trope. Workers aren't literally bees, but it suggests how fast they move.
Trope refers to different types of figures of speech, such as puns, metaphors, and similes. Each has its own particular structure, but in each case the actual meaning is different from the literal, dictionary sense. Trope is also used in a more general sense to describe a convention that you can easily recognize and understand because you've seen it so often. For example a TV cop show might use the trope of police vs. thieves to talk about larger issues.