Caul fat is a fatty membrane that's around the stomach or kidneys of veal. It's white (or clear), completely tasteless, and pure fat. It also looks like alien flesh or a kind of net with a clear "plastic-paperish" skin between the part resembling a net. You wrap is around something (usually a roast) to both hold everything together, and to baste the meat while it cooks. An example of this would be if you had a roast and you wanted to have a bay leaf or something of the sort cook literally with the meat. The caul fat all cooks away, so nothing weird looking is on the meat once complete.