Definition of an Idiom: a group of words that accompany an expression that does not apply to the literal meaning, but contains another, hidden meaning.
Examples of idioms:
It's raining cats and dogs outside!
Translation: It's raining really hard outside!
That last exam I had was a piece of cake.
Translation: That last exam was very easy.
Ashley hit the nail on the head when she described Tony's personality.
Translation: Ashley was right on when she described Tony's personality.