What's an Idiom?

What is an idiom?

An idiom is an expression whose meanings cannot be inferred from the meanings of the words that make it up.

For example the term "crying over spilled milk" doesn't mean to literally cry over spilled milk. It means not to cry over or worry about something that has already happened. 

"Letting the cat out of the bag" doesn't mean to let a real cat out of a bag. Rather it means to share information you have been keeping secret. 

So as you can see idioms cannot be explained by the words within the phrase. They stand for other meanings. Try to come up with some other idioms you have heard.