Like dogs, cats say things with their tails. What they’re saying depends on the particular tail motion.
If the cat’s tail is doing a thrashing motion, the cat is angry. If your cat is near you doing a quivering motion with her tail, it’s an expression of affection.
When your cat is simply relaxing and the tail is doing a leisurely swishing motion, that signals contentment. I liken it to a relaxed person swinging his foot back and forth, the way we humans do sometimes.