If it's going to rain cats and dogs later today, you'll want to bring an umbrella with you. To rain cats and dogs is to rain extremely hard, or to pour.
When it rains cats and dogs, you tend to get very wet. The saying presents an interesting image, of animals falling from the sky, that doesn't seem to have much to do with torrential rain, and experts aren't certain about its origin. The most likely — and least appealing — theory is that rain storms in 17th century England would carry the bodies of dead animals through the dirty city streets, inspiring people to say, "it's raining cats and dogs."