The molecules of surfactant used in dryer sheets have unusually large fatty sides, and their water side is good at grabbing onto fabrics. Mix it with your clothes, and the water side locks the molecule in place while the oily side points outward, coating the surface of every piece of clothing with an extremely thin layer of oil. You can feel this oil yourself by rubbing one of the sheets between your fingers. This oil allows your laundry to tumble around without the static buildup that ordinarily occurs when your clothes rub together. Your laundry comes out softer too, because it’s just that little bit greasy!