because fat causes the gallbladder to contract, forcing bile into the digestive tract to emulsify the fat so it can be broken down into fatty acids and absorbed. The danger is that the gallbladder will shoot out one of those stones which will then block the common bile duct, the one that branches off from both the liver and gallbladder. If that happens, you need emergency surgery, no choice, or you'll die as bile backs up into the liver and that organ eventually fails. You'll want the surgery, though because it HURTS.
You can eat those apple skins. They're not fatty and they don't enter the bile duct. I'd avoid that low fat sour cream and use NO fat yogurt instead. No, it's not the same, but it'll dress up a plain baked potato until your surgery is done. You're going to have to eat stuff you wouldn't ordinarily consider (canned fruit in sugar syrup comes to mind) and forgo stuff you would, like anything cooked with cheese, olive oil, or full milkfat dairy products, or even soy milk. Check those labels, you want as little fat as possible. Make sure canned tomato sauce you use with pasta is not loaded with cheese or olive oil. If there's any oil in it, use it sparingly. You can throw all sorts of veggies in it and simmer it for a while, just don't sautee those veggies in oil. Low fat deli meats like turkey are generally OK, and you'll get used to turkey and lettuce and tomato between two slices of bread, no mayo.
Remember, you're just eating to give yourself energy so you can stay alive until that surgery. Your usual nutrition is going to have to fly out the window for now.
Gallstones usually don't look all that impressive when they're removed, sort of like yellow gravel swimming in greenish black bile. They can cause horrible trouble until they are, though.
Here is everything you ever wanted to know about gallstones, at great length:
http://www.emedicinehealth.com/gallstones/article_em.htm