When something is cliched, you've heard it a million times before — it's overused and unoriginal. Your English teacher might advise you to avoid cliched sayings in your essays.
You'll know when a phrase is cliched, because it will sound extremely familiar. "Every cloud has a silver lining," "think outside the box," and "there are plenty of fish in the sea" are all cliched. These are fine to use in casual speech, or in an ironic way, but you shouldn't use them in formal writing. Many people mistakenly use the noun cliche as an adjective, instead of cliched.