Idioms in English

An idiom is a phrase that is common to a certain population. It is typically figurative and usually is not understandable based solely on the words within the phrase. A prior understanding of its usage is usually necessary. Idioms are crucial to the progression of language. They function in a manner that, in many cases, literal meanings cannot. Idioms are words or phrases that aren’t meant to be taken literally.
Webster's New World adds "[It] has a meaning that differs from the literal meaning of its parts taken together." For example, if you say someone has “cold feet,” it doesn’t mean their toes are actually cold. Rather, it means they’re nervous about something.