An idiom is a group of words that has a special meaning that may not be clear when you look at each separate word. Idioms are unique to a language and are used by native speakers in everyday conversation. For example, "under the weather" is an idiom that means 'slightly ill and not as well as usual'.
If you don't understand idioms, you might interpret them literally and misunderstand the intended meaning. Learning idioms helps you avoid confusion and accurately interpret what others are saying.