IT RAINED CATAND DOG WHAT FIGURE OF SPEECH
There are different interpretations of how this saying came about, but generally it just means it's raining heavily -- lots and lots of rain!
https://www.google.com/search?q=idiom+raining+cats+and+dogs&oq=idiom+%22raining&aqs=chrome.1.69i57j0l5.8137j0j7&sourceid=chrome&ie=UTF-8
PS. It might be a metaphor, but check here to be sure after you've read about the origin of the old saying.
http://www.tnellen.com/cybereng/lit_terms/
The phrase "it rained cats and dogs" is an example of a figure of speech called an idiom. An idiom is a commonly used expression that conveys a figurative meaning rather than a literal one.
To understand the meaning of this idiom, we don't take it literally as if it is actually raining cats and dogs. Instead, it is used to describe very heavy or intense rainfall. It suggests that the rain is falling extremely hard, similar to the way cats and dogs might fall from the sky if it were possible.
Idioms can be a bit tricky for non-native speakers or for those unfamiliar with the specific expression. So, understanding idioms often requires prior exposure to the language and its sayings.