A common expression used to mean something else?

You're asking for an idiom! An idiom is a phrase that has a figurative meaning that is different from the literal meaning of the words. Here are a few examples:

Idiom: "Kick the bucket"

Literal Meaning: To physically kick a bucket

Figurative Meaning: To die

Idiom: "Break a leg"

Literal Meaning: To break a bone in your leg

Figurative Meaning: Good luck

Idiom: "It's raining cats and dogs"

Literal Meaning: Animals are falling from the sky

Figurative Meaning: It is raining very heavily

Do you want to know more about a particular idiom, or are you looking for a specific example? Just let me know!