Hi Julia, an idiom is a figure of speech or an expression that is usually culturally specific in its meaning and usage. The meaning of an idiom is usually something you'd only know if you come from the same culture as the idiom and know its meaning from hearing it used all your life, like, "Cat got your tongue?" or, "I'm all ears." Some idioms are shared by more than one culture, like, "Long time, no see." has the same meaning and usage for the Chinese, Americans, and some Native Americans.

"He sucked the big kumara" - it's (mostly) English, but it's New Zealand idiom. Unlike several of the other examples given here, that is one you perhaps wouldn't know. It probably is as unintelligible to you, as many more standard idiomatic phrases that we use without thinking are to English learners.

Have you ever said something like this, I have never tried sky diving but I am willing to *give it a shot?* Or, I can't go out today because I feel *under the weather.* Or, *He is pain in the ass.* All these phrases within the asterisks are called idiom. An idiom is a phrase of which meaning can not be determined just by looking at the words.