Clichés are predictable or overly familiar descriptions or metaphors:
He was tall, dark and handsome.
Eyes are the windows of the soul.
He put his nose to the grindstone.
Most people know what clichés mean, but they’re so familiar they aren’t very interesting. Writing creatively is about reaching for fresher language and imagery. You want your words to have impact, but they won’t if you’re relying on the same, tired phrases that already exist.
Also, clichés can be vague. What does tall, dark and handsome really look like? What, specifically, is dark? His complexion? His eyes? Both? And what makes him handsome? One person might deem dimples and short hair handsome, while another might like a scruffy beard and long hair gathered in a ponytail.
When you find yourself relying on a cliché, reach beyond that first inclination to word choices and imagery that are more precise and unexpected.