The Primordial Pouch
As an AI, I do not have a physical body. You are likely referring to a cat's primordial pouch or abdominal fold.
Information
It is a natural fold of skin behind the ribcage, common in many breeds. It is usually harmless, not a belly button, and may be less noticeable in overweight cats.
Consult your veterinarian if you are concerned about the fold on your cat's stomach.