A cliché is a phrase, plot point, character, etc., that is overused and overfamiliar. Clichés make your writing seem boring, unoriginal, and simplistic—all things that good writers strive to avoid like the plague (see what I did there?) in order to keep readers focused on the story.
It’s fairly common for clichés—especially clichéd phrasing—to sneak into even the best writers’ work. Seeking them out and eliminating them is a normal part of the work of revision, but there are two main difficulties. First, one has to recognize clichés. This is especially problematic for newer or younger writers, because clichés become obvious only through exposure to them over time. The second main difficulty is that they are often invisible, because they make sense—they fit. This means that that finding an effective replacement can also be very hard. (Note: When finding ways to replace clichés, be careful to avoid awkward, confusing, or convoluted phrasing.)
Among the many examples of clichés in phrasing are: dumb as a post, slept like a log, my heart raced, all talk and no action, the grass is greener on the other side, last ditch effort, light as a feather, rain on my parade, reinvent the wheel, cut to the chase, and so, so many more. Here’s a great site with tons of examples: http://clichesite.com/alpha_list.asp?which=lett+1. I’m not suggesting these should never be used. In fact, they’re often entirely appropriate in dialogue. But if your writing has too many of these clichés, your work will be seriously weakened. It’s worth the effort of learning to recognize them, and at least reducing the frequency with which you employ them.
Clichéd plot points can be even more difficult to notice, because they depend on you, the writer, being familiar with your genre. You might come up with a brilliant story idea for a murder mystery, for example, but if you haven’t read widely in that genre, you might discover–after you’ve put a lot of time into writing, revising, and submitting—that you’ve used a plot that’s been done and done again, and that seems tired (old hat!) to folks who do know the genre. If you’re committing to writing in a particular genre, it makes sense to read widely in that genre (of course). But in the meantime, find a good writing group or set of friends and associates who you can bounce ideas off of. They can save you a lot of time.
Similarly, it’s easy to fall into character clichés or stereotypes, if you’re not careful. One I wrote into a story myself is the female healer. Once I came to know that genre better, I changed her to a blacksmith, which worked out even better for the story. Subverting reader expectations for characters can make your stories more interesting, in part because you have to work harder to make characters more complex—more three-dimensional.
There are many great resources for understanding and avoiding story and character clichés. A great general source, though it’s aimed specifically at television, is the website TVTropes.org http://tvtropes.org/. In spite of what their intro screen says, many tropes are, indeed, clichés or stereotypes, and the site is well worth extensive exploration. There are also a number of sites that identify genre-specific clichés to avoid, including http://www2.silverblade.net/cliches/, the sci-fi cliché section of the TV Tropes site, and http://www.strangehorizons.com/guidelines/fiction-common-horror.shtml. Diana Wynne Jones’ book The Tough Guide to Fantasyland is also worth a read if fantasy is your field.
Now, go learn, and then write, revise, repeat. And don’t forget: tomorrow is a new day, always look on the bright side, and the early bird gets the worm.