Hi 李菲菲, This is an English idiom to express the meaning of a very heavy downpour. Some idioms have interesting origins. There is a saying that in the past, domestic animals such as cats and dogs would make the roofs of houses their homes. Unfortunately, when there is a heavy downpour, these poor creatures would be washed off the roofs with the rain. They appear to be falling together with the rain, thus the idiom "raining cats and dogs". Reference URL: https://english.stackexchange.com/questions/14273/the-etymology-of-the-phrase-its-raining-cats-and-dogs

Heavy rain. It is raining so hard that if feels like cats and dogs are falling out of the clouds. It's nonsense if you take it literally. It's an idiom that you just have to get used to when its raining heavily in an English dominant country.

It's an expression used when it's raining extremely hard. At least, that's the meaning where I live in in the USA.

您好，李菲菲。 Lance的解释超好。 我所理解的，To rain cats and dogs 汉语就说倾盆大雨。 希望有帮助， 玲玲

it means to rain very heavily, according to Cambridge Dictionary.