An idiom is a phrase or expression that has a figurative meaning, often completely different from the literal meaning of the words. They are specific to a language and culture, and often don't make sense if you try to understand them word for word.
Here are some examples of idioms:
* "Kick the bucket" means to die.
* "Spill the beans" means to reveal a secret.
* "Break a leg" means good luck.
Literal means the actual or exact meaning of a word or phrase.
Since idioms are *figurative*, they are by definition *not* literal.
Perhaps you meant to ask about figurative language? That encompasses idioms, metaphors, similes, and other devices that use language in a non-literal way.