What does it mean if your cat does not have a tail?

Unless it has been in some sort of accident, it means your kitten is a bobtail, which is the common name for two specific breeds of cat: the Japanese bobtail (usually white) or American bobtail (can be any color/color combination).

2nd Answer:

There is the breed known as the Manx and they have not tail. If there is any Manx blood in her background genes, then that is the reason she has no tail, other than an accident.