A cliché is any word or phrase that is used too much. (By extension, it can also mean an oft-repeated situation, scene or pattern in a work of art like a film, an opera or a novel.) It is most often used about a figure of speech that is used unthinkingly, or with an image that is no longer fresh and vivid. So good writers should avoid clichés. Students may find that their teachers regard clichés as evidence of inadequately considered writing.
Cliché was originally a French word. The acute accent over the '-e' (é) - which should always be used - shows that it is pronounced as two syllables. In Britain, we prefer to attempt the French pronunciation with a very slight stress on the first syllable ('KLEE shay', IPA: /ˈʃkliː ʃe/. American speakers stress the second syllable ('klee SHAY', IPA: /kliː ˈʃe/. This is regarded as incorrect in British academic circles.