Means it is raining VERY hard.

A literal explination for raining cats and dogs is that during heavy rains in 17-century England some city streets became raging rivers of filth carrying many dead cats and dogs. The first printed use of the phrase does date to the 17th centurey, when English playwright Richard Brome wrote in The City Witt (1652): "It shall rain dogs and polecats." His use of "polecats" certainly suggests a less literal explination , but no better theory has been offered. Other conjectures are the the hyperbole comes from a Greek saying, similar in sound, meaning "an unlikely occurrence," and that the phrase derives from a rare French word, catadoupe ("a waterfall"), which sounds a little like cats and dogs. It could also be that the expression was inspired by the fact that cats and dogs were closely associated witht the rain and wind the Northern mythology, dogs often being pictured as the attendants of Odin the strom god, while cats were believed to cause storms. Similar colloquial expressions include it's raining pirchforks, darning needles, hammer handles, chicken coops, and men.

As correctly stated, this is a literal phrase dating from 17th century England. Back in the day, peasants used what little land they owned for crops and such, so could not afford to keep cats and dogs on their land. As a result, people used to keep their animals on the thatched roofs of their cottages. When it rained heavily, the thatching became very perilous and slippery, causing the cats and dogs to fall off!

We say it's raining cats and dogs when there is a heavy downpour. It simply means a heavy rain.

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"

The heavy rain.
The heavy showers with air.
The rain which irritate peoples by any mean.

A lot of rain or raining hard