Idiomatic language refers to many words or phrases that are a familiar and everyday feature of our language. Idioms are a part of the comfortable, conversational style of language we use daily - but to a foreigner, idioms are difficult to understand because their meaning is very different from the literal meaning of the words that make them up, e.g. 'He wants his pound of flesh.' 'You scratch my back, I'll scratch yours''He's a pain in the neck!', etc. Each of these are idioms - or idiomatic phrases.