Idioms are phrases or expressions that, most commonly, possess a figurative meaning (that is to say, its meaning is not literal). A good example is “break a leg”, a phrase that is said to an actor before performing on stage, and it means: good luck.
Although many idioms are quite popular and used consistently within a whole country, there are others than are more localized and used only in certain regions, which means that an idiom may not necessarily be understood by everyone, but rather only by those who are familiarized with it because of their geographical location.
Idioms are words that mean something, but not literally. It's raining cats and dogs is a popular idiom. The reason it is false is because idioms are usually only popular in the place of their creation.
The theme of a story is important because a story's theme is part of the reason why the author wrote the story. The author has a message he wants to share with readers, and he uses his story as a way to get that message across.
