Final answer:
The sentence 'It is raining cats and dogs outside' means that it is pouring rain outside. This is an idiom in English that means it is raining very heavily.
Step-by-step explanation:
The sentence 'It is raining cats and dogs outside' means that it is pouring rain outside. This is an idiom in English that means it is raining very heavily. The phrase 'cats and dogs' is not meant to be taken literally, as it does not mean actual animals are falling from the sky.
Learn more about Meaning of an idiom in English