This is an idiom, meaning that it's raining very heavily. It's a fun way to describe a downpour.
The origin of the idiom isn't fully known, but some theories suggest it came from:
* Stray animals: In the past, stray animals might have sought shelter in homes during heavy storms, giving the impression of animals being carried in the rain.
* Old English: The phrase "catadupe" meant "waterfall" in Old English, which might have evolved into the idiom.
* Misinterpretations: It could be a misinterpretation of phrases like "raining pitchforks" or "raining fire and brimstone," which also described heavy downpours.
So, while it's a fun saying, you don't need to worry about dodging felines and canines when the weather forecast calls for "cats and dogs"!