You are likely referring to a cat's "primordial pouch" or "abdominal fold". This is a fold of skin that appears as a flap on the cat's belly, right behind the ribcage.
Here's some information about it:
* It's not harmful: The abdominal fold is a natural feature in many cat breeds, especially those with thick fur. It's usually harmless, although it can collect dirt and debris.
* It's not a belly button: It's often mistaken for a belly button, but the cat's actual belly button is a tiny scar on the belly, located further down.
* It's not always there: Some cats have it, and some don't. It can also become less noticeable as the cat gains weight.
If you are concerned about the fold on your cat's stomach, it's always best to consult your veterinarian. They can assess your cat's health and make sure the fold isn't causing any issues.