If you're ever frustrated because you can't communicate verbally to your beloved kitty, realize that just because she can't speak to you per se doesn't mean that she can't express her feelings eloquently. In fact, a cat's tail often relays feline messages purrfectly.
Some cats can and do occasionally walk with their tails down, although it usually isn't a very happy or content gesture. According to the Humane Society of the United States, walking with a lowered tail can be a sign of uncertainty and insecurity. Perhaps your cat is used to being a big deal at home and is feeling like a little guy now that you adopted a new, younger kitten and everyone is giving her all of the attention. Maybe he's concentrating on all of the birds fluttering outside of the window and isn't certain about how exactly he's going to go after them!
When a feline is walking around with his tail down, it can also mean that he's feeling rather nervous about something at the moment. He's just not in his element, poor thing. Maybe you brought his carrier out and he's starting to feel panicky at the idea of a dreaded routine veterinarian visit. Perhaps your suitcase is on the ground and he knows what that means -- several days of utter loneliness without you in his daily life, aww.
If your cat's tail is not only down, but is also placed in between his hind legs, he's very likely extremely frightened. A cat in this mode probably is scared of a predator attack, whether that "predator" is another cat in your household or a new guest in your home -- yikes. Be careful when you're around a cat feeling this way. The lowered tail is often a defensive stance. That timid fear could very quickly lead to full-out aggression, so stay out of his path and allow him some time to cool down.
Context can mean a lot when it comes to deciphering the meaning behind cat tail placement. For example, a cat walking with his tail in between his legs can indicate a severe state of fear. However, it can also point to submission, a very different emotional state. When a cat feels overpowered by someone or something, he may throw in the towel by making a visible admission of defeat. Unlike a scared kitty, this one is not about to attack, and probably won't be growling or hissing, either.