Idioms are groups of words that explored separately have different meaning, than when put together, and form an idiom! An idiom conveys a figurative meaning, and exploring the words individually wouldn't give you the meaning the idiom does. All cultures and languages have phrases like these that convey certain advices, wise thoughts and attitudes, that can also tell you a lot about the cultural preferences and values. Usually native speakers use them frequently, so it's important to get familiar with at least the most commonly used idioms in the society you want to reside, in order to merge easily and understand the native people. Idioms can also be so debilitating to language learners because as I said, given the words separately have a different meaning, it's not so easy to remember the idioms by heart, and also many conversations can lead to confusions because if you take them literally, they can sound illogical and irrealistic.
"I don't like when people judge the book by its cover."
However, many times foreigners learning the language would believe anything, so they might also believe that what they assumed the construct of words was about, is correct. So it's advisable and also can be very amusing to many people, to check for these kind of intricacies of a culture when learning a new language.
What do You think? Do you recognize any of the idioms mentioned on the page I shared? How do you like them? Is it hard for you to learn idioms? Or you like getting into and memorising expressions like that in a language ?