What does raining cat and dogs mean?

"Raining cats and dogs" is an idiom, meaning raining very heavily. It's a hyperbole, meaning it's an exaggeration used for emphasis.

The phrase doesn't actually refer to animals falling from the sky!

Here are some examples of how you might use it:

* "I can't go out, it's raining cats and dogs!"

* "We had to cancel our picnic because it was raining cats and dogs all day."