If it is raining cats and dogs,
it is raining heavily. This is a rather
old-fashioned expression, and you are probably more likely to read this
methaphor in a book than to hear it said. Now, people are more likely to say,
"it's pouring hard" or "bucketing down" to mean raining heavily.
"Did you enjoy the picnic yesterday?"
"No! We had to eat in the car. It was raining cats and dogs!""