A cliché is a phrase that is symbolic or figurative rather than literal; for example: I am over the moon. Really? Clearly not, unless you are in a space ship, in which case it is a literal situation and therefore not a cliché.
A cliché can be an idiom, as in the example above; a metaphor [it was the final straw]; a simile [she sank like a stone]; or a proverb [knowledge is power].
At one time, eons ago, it was considered natty to use such phrases in speech and in writing. The concept was still new and they sounded clever.