The phrase "raining cats and dogs" is an idiomatic expression that means it is raining very heavily. It is used to describe a torrential downpour or a heavy rainstorm.
In the past, when houses had thatched roofs, this is like the Middle Ages, small animals like cats and dogs would seek shelter in the thatch. During heavy rains, the animals could be dislodged and would fall from the roof, giving the appearance of "raining cats and dogs."
"Raining cats and dogs" is now a common idiom used to vividly describe a heavy rainstorm, emphasizing its intensity. If you did see animals falling, that would be weird.
