An idiom is an expression where the words together have a meaning that is different from the dictionary definitions of the individual words. Like:
"pull someone's leg" [joke]
"feel under the weather" [feel ill]
Why learn idioms?
What makes learning idiomatic language important is the fact that much of the everyday speech is based on idioms. For one thing, knowing and being able to recognize them in spoken speech will help you understand a native speaker better. Once you master them and become more fluent in English, you will be able to communicate better, as well.
Understanding idioms can be a difficult task for someone not native of America. Here are several popular idioms used in American culture, and the literal meaning behind them.