Idioms are fixed lexical chunks with a specific meaning that often cannot be worked out from analysis of the literal meaning of the words used, although those words do have a literal meaning and, in some cases, once the meaning of the idiom is known, an analogy can be made - as in the first, second and third examples below.
Examples :
- to bark up the wrong tree
 - to beat around the bush
 - to hear something on the grapevine
 - to let the cat out of the bag
 - to take something with a grain of salt
 
Other idioms are more transparent in meaning, as in eg It's pouring with rain / It's tipping it down.
See also : My teacher is an old cow : Teaching figurative language, should we or shouldn't we?
Further Reading
Further Reading
Lewis, M. Implementing the Lexical Approach, CENGAGE ELT