That's called a fat pouch, also known as a fat sack or fat pack. It only occurs in cats who are over weight. My Abyssinian has it, and I've had to put him on a diet. My Tabby cat doesn't have it. Don't worry about it too much, but try to get her to lose a bit of weight.
Note, not all cats have it. It's not normal, only overweight cats have it. It won't affect her health, but carrying that excess weight around isn't good for cats, she might end up having joint problems or arthritis when she gets to an elderly age.