Why idioms? We derive meanings of a text/phrase/clause from the words in it. However, sometimes it is improper to deduce or get the meaning from a text by translating word by word.
What is an idiom?
An idiom is a group of words. In the usage, we can not get the meaning merely by translating the individual words (e.g., rain cats and dogs, see the light ). When we heard an idiom, e.g. It is raining cats and dogs, it doesn’t literally mean that there are some cats and dogs falling down to the earth from the sky. This idiom shows a kind of rain, that is when the rain starts and stops.