Trope: in literature, a familiar and/or often used symbol, style, character, theme or device.
Tropes are devices that a writer can reasonably rely on as being present in a reader’s mind and expectations. On the whole, tropes are not clichés. The word clichéd means "stereotyped and trite." (In other words, dull and uninteresting.) A trope is a convention. It can be a plot trick, a setup, a narrative structure, a character type, a linguistic idiom... you know it when you see it. Tropes are not inherently disruptive to a story; however, when the trope itself becomes invasive, distracting the reader rather than serving as shorthand, it has become a cliché.
Steve Feasey, author of Changeling, says, “A trope is something that immediately resonates with a reader: they may have come across it before in another form of entertainment – a book or film. As a writer your job is to take those tropes – those recognized elements – and rework them so they fit within your world, the one you are creating for your reader. It's very rare that you get a truly original monster.”
Tropes are tropes for a reason: they generally work. Yeah, some people rail against them as being predictable, cliché, too easy, but they are markers, a signal as to what kind of story we have encountered, what we might expect if we proceed further. Your goal as an author should be to know the tropes of your genre, twist the story, surprise the audience, and turn the cliché upside down or inside out.
It comes down to rhetoric: the art of persuasion. A story must persuade a reader’s acceptance of credibility, setting, characters, and conflicts.
Check out the Write it Better section along the right side of the blog. I’ve added several links that include a comprehensive list of examples and explanations of several types of tropes: characterization, emotional, internal conflict, attraction, etc.
Explore them. Twist them. Have fun with them.