After selecting my size, I went into the dressing room and slipped the dress on. It was a very pretty dress ... but ... wait ... what? what is this? What???

Can someone PLEASE explain to me WHAT this is? It was almost like a pocket, but it went all the way through. It laid flat, unless I took a step and then it did this weir puckering thing where you could see through to the other side. Is it a hand warmer?
I know I'm not a fasionista, but seriously, what's up with this?