Idioms are words which have a different meaning from the individual meanings of each word.People use idioms to show their proficiency in English.
Raining cats and dogs: This phrase is an idiom. It means it is raining heavily.
Let us consider a situation related to this. When your friend had invited you for dinner and you were late because it was raining heavily. He calls you to find out the reason for the delay .In such a situation say:" It is raining cats and dogs . I am afraid I won't be able to make it today"
When I went for shopping yesterday, it was raining cats and dogs.
How can I go out now? It has been raining cats and dogs since an hour!