It’s called a Dewlap. This is mostly fur and flabby skin. It’s mostly found on doe’s and sometimes overweight bucks. Doe’s pull fur from this flap to line their nests for their litters. This way their litter can stay warm. The dewlap stores a little energy reserves as well which is used after kindling. Once a rabbit doe kindles you’ll see the dewlap shrink and get noticeably smaller.