This isn’t really a medical term, but I think women are referring to the skin overhang of your tummy. Essentially, the skin in your groin is quite thin and doesn’t stretch very much, even when you gain weight. The ligament at the bottom of your abdomen doesn’t really expand like your upper abdomen, so it’s not really stretching the skin in that part. Above that, though, your skin is stretching. So, when you lose weight, that lower skin sort of acts like a tight belt, and the skin above hangs over the top of it. A tummy tuck will address that excess skin that hangs over your groin, causing the apron.