"It's raining cats and dogs" means that it's raining very heavily (big drops of rain). I don't know where the saying comes from, and it must be sort of strange for a non-native speaker, but it makes you think of cats and dogs falling from dark clouds - heavy rain indeed!

Raining cats and dogs is an expression depicting a heavy downpour of rain.

It means it's raining heavily. "I got soaked outside - it's raining cats and dogs!"