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
IT RAINED CATAND DOG WHAT FIGURE OF SPEECH
2 answers
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/
http://www.tnellen.com/cybereng/lit_terms/