An anti-scald device can also be called a pressure-balancing valve. It is a fitting for a single-control faucet that includes a piston that will respond to changes in water pressure in the line so that the water temperature stays constant.
The valve prevents sudden changes in water temperature — either a scalding rush of hot water or a chilling burst of cold water. Anti-scald devices make sense in tubs, showers and sinks regularly used by children, elderly adults or persons with nerve diseases or other disabilities that make it difficult to sense these changes quickly or to respond or react to sudden temperature changes quickly.
Some localities require installation of anti-scald devices in all bathroom faucets to protect children in particular. If you want to include the valves in new faucets as part of your remodel, be sure to tell your contractor or plumber early in the process of selecting your faucets.