If you say it's raining cats and dogs, you mean that it is raining very heavily.
Note: There are several possible explanations for this expression, but none of them can be proved. It may refer to the days when drainage in towns was so poor that cats and dogs sometimes drowned in heavy rainfall. Alternatively, `cats and dogs' could be a corruption or misunderstanding of the Greek word`catadupe', meaning `waterfall', so the expression would originally have been `it's raining like a waterfall'. The origin may also be in Norse mythology, where cats and dogs were sometimes associated with the spirit of the storm.