For questions about commonly-used storytelling devices, such as character stereotypes, plot devices and other tricks of the trade.
A "trope" in literature was originally defined as simply "a figure of speech", but in more modern parlance it has the meaning of "a significant or recurrent theme or motif". Tropes are essentially patterns of storytelling that are identifiable and relatable; their purpose is to make the audience subconsciously assign additional intuitively-known values of the trope to its usage in a specific story, so the writer doesn't need to explain himself any further.
For instance, when a broad-shouldered, athletic man in spotless bright silver armor rides in over a hill on horseback to dramatic uplifting music, the viewer knows this guy is the main hero of the movie who will ultimately defeat the guy in black in a fiery, spectacular duel in the depths of a moldy castle. The audience knows all of this before anyone on-screen opens their mouth, because they've seen it before; it's the eons-old "Knight in Shining Armor" trope.
If the trope unfolds exactly as described, many viewers will kick themselves for not leaving the theater or putting the book back on the shelf as soon as they saw this; this trope is "discredited", so old and overused that the audience sees the cliche coming a mile away and has a negative reaction to it. However, writers are cunning, if ultimately unoriginal, and this guy might turn out to be the "Anti-Hero", a guy with a good guy's name, position, character traits and goals, but who eventually ends up being the bad guy by either abusing his position or by acting on some key mistakes of fact, until he's doing exactly the opposite of saving the day, and someone else has to defeat him in order to actually save the day.
Practically every character, setting, goal, tool or other plot device in modern fiction can be categorized as a trope or known variation of a trope (such variations being known as meta-tropes, such as the "trope inversion" of the Knight in Shining Armor being the Anti-Hero), because it was originally an excellent original idea that has since been ripped off in other works of fiction. The use of a trope by an author, in itself, is neither good nor bad; trying to avoid tropes altogether tends to lead to demonstrably bad fiction writing such as overexposition, unrelatable characters, implausible plotlines etc. It's merely very good, as a writer, to know what tropes are being used, and why.