Are you word smart? Then you know an idiom is a word or phrase that has developed a meaning that is different than the meaning of the words the idiom is constructed with? Right? It's not difficult. Think of phrases such as "raining cats and dogs," "kick the bucket," "straight from the horse's mouth," and "once in a blue moon."
Now, we've never witnessed cats and dogs actually falling from the sky, we're pretty sure that kicking a bucket wouldn't kill you, horses can't talk, and the moon isn't actually blue. The thing is that these colorful phrases are idioms; they are phrases that developed over time to have meanings other than what is literally meant.
All languages have idioms. They are one of the components of language that make learning a language other than your native tongue difficult. If you told a new English learner that it was raining cats and dogs outside, he or she would likely be very confused... or think you were seeing something.
Idioms are fun. Take this quiz to find out if you know common phrases better than most everyone else.