When a cats tail is down, it can indicate that the cat is feeling defensive, submissive, nervous, or scared. However, it is important to note that some cats may naturally carry their tails down when they are in a neutral or relaxed mood. The position of the tail, as well as its movement, can give clues about a cats mood and intentions. For example, a tail that is bristled or puffed up can indicate that the cat is agitated, angry, or afraid, while a tail that is held straight out behind the cat can indicate that the cat is focused on an object. It is important to observe other body language cues and the context of the situation to accurately interpret a cats behavior.