What do you mean it's raining cats and dogs?

What does this mean?

It's raining cats and dogs

It means:

It is raining cats and dogs outside.

Rain cat and dog is a fixed usage, which is equivalent to an idiom in China.

It is raining cats and dogs.

This phrase first appeared in17th century, and its origin has not been verified. It is generally believed that its origin may be related to the underground drainage system used in17th century. At that time, the drainage system was very simple and the drainage capacity was limited. Once it rained heavily, the sewage from the underground drainage ditch crossed the river, and not only the garbage, but also the dead cats and dogs spread with the sewage. Therefore, dogs and cats are associated with downpours. From this comes the phrase downpour. Visible, cats and dogs rain does not mean that cats and dogs fall from the sky, but heavy rain leads to poor underground drainage, and dead cats and dogs in sewers float with sewage.

On the other hand, when there is a storm, it looks like a cat and a dog are fighting.

Some people think it comes from Nordic mythology. Nordic people think that cats affect weather changes, while dogs represent the wind. So cats and dogs can't escape the heavy rain