The Latin prefix dis- means apart, asunder, away, utterly or having reversing or opposite forces or ideas, and also classified as simply not or not any. Like for example, discontent, dishearten, discomfort, disrepair, discredit, disbelief, dislike, disown, disability, etc.
so, the first sentence informs you that it's raining cats and dogs. when you add the comma after raining, the meaning completely changes. the speaker is telling the cats and dogs that it is raining.
