A slather (sometimes referred to as a binder) is simply something applied to the surface of the meat that helps the dry rub stick (or bind). A slather can be as simple as water, oil, mustard, or even tallow (made from brisket trimmings). I personally find using a slather necessary for brisket, as the rub sticks just fine. But if you want to get some extra stickiness, just apply a little slather right before you sprinkle on the dry rub.