You probably know someone who's really bad with idioms. That's the person who tells you that you should get your hands wet with a new project or that their friend's most recent betrayal was the last camel's straw.
Idioms can be some of the toughest things to learn about a new language. Think about it. If English weren't your first language, what would you think someone meant when they told you to go fly a kite? Probably that they wanted you to, you know, go buy a kite at the toy store, walk outside, and fly it.
That's because the words that make up idioms mean one thing on their own, but when you put those words together, they mean something totally new.
P.S. If you've reached the end of this definition and you're totally confused, that probably because you're thinking about the word "idiom" in another way. Idiom can also mean language—as in the local language of a group of people. William Carlos Williams had plenty to say on that kind of idiom.