Polydactylism is mostly genetic. The wolf and most dogs have 4 ground-contacting toes on each foot + one dewclaw on each front leg (total of 18 toes). read more
It’s just a genetic defect, as other users stated. It’s probably nothing to worry about. Some genetic defects suggest further issues in the dog. For example, Dalmations born with blue eyes are usually deaf. read more
Most breeds have 5 toes on each fore leg and 4 toes on each hind leg. It may be a genetic factor. If she has trouble walking on it (limping, dragging) take her to a vet where they may have to remove it. read more