It's Raining Cats and Dogs means:
A heavy downpour, rain coming down very quickly and hard.
Example of use:
"There's no way they'll be playing at the park, it's raining cats and dogs out there!"
There is no definite origin of this popular phrase. Its first recorded use was in a collection of poems that was published in 1651. Jonathan Swift introduced the phrase to the mainstream in 1738 when he published a satire on the speaking manner of the upper class called "Complete Collection of Genteel and Ingenious Conversation".