1. An overused expression, something that is said a lot that has become some common, it no longer really has any relevance or is even noticed in conversation. Phrases such as "to this day" or "next thing I knew" are examples of such a cliches, and you often say these phrases without noticing you are doing so.
2. An idea with a different meaning from its literal meaning. For example, the phrases "sweaty palms" or "twinkling eyes" have come to mean more than the fact that your palms are just sweaty or that your eye's have a twinkle. When you say someone has sweaty palms, everyone knows you mean "he is nervous" because the expression has become a cliche.