An idiom may be defined as a group of words whose meaning cannot be understood from a literal interpretation of the group of words.
Consider this math problem: "1 + 1 = ?"
Idiomatic English: The answer is a piece of cake. The answer is "2".
Simple English: The answer is easy. The answer is "2".
English is an idiomatic language.
The English language contains thousands of idioms.
Native speakers of English use and understand idioms naturally and automatically.
Students of English need to become adept with using and understanding idioms to be considered fluent in English.